Sequel Number Snarl

Basically, this happens when the numbering of sequels gets really, really complicated. A Sequel Number Snarl often occurs when Sequels and Interquels start filling up the chronology, but then gets fouled up by the presence of an Oddly-Named Sequel 2: Electric Boogaloo or two (or five...) whose title doesn't include a number, is added to the series.

A subtrope of Numbered Sequels.

Anime and Manga

 * Full Metal Panic!, followed by the interquel Full Metal Panic? Fumoffu, followed by the sequel Full Metal Panic! The Second Raid.

Film

 * The Ring series is somewhat baffling.
 * A 1995 film, named Ring and rereleased as Ring: Kanzenban
 * Hideo Nakata series: Ring, Rasen, Ring 2 (which ignores the events of Rasen) and a prequel Ring 0: Birthday
 * American films The Ring, Rings (a short DVD-only film) and The Ring Two
 * Korean film The Ring Virus
 * The Rambo series goes First Blood, Rambo: First Blood Part II, Rambo III, and then Rambo (also known as John Rambo or Rambo IV in certain countries).

Live-Action TV

 * Power Rangers gained a bit of this in terms of season numbers when Disney decided to Recut part of the first season. Officially, the Recut is Season 18, but fans are reluctant to name glorified reruns as a full season and consider the following span of new episodes, Power Rangers Samurai, as the actual Season 18. The impending adaptation of Kaizoku Sentai Gokaiger is affecting fan perception as well, as it's a Milestone Celebration season (35 years of Super Sentai), would be the 20th year of Sentai adapted for Power Rangers, and would air the year of Rangers' 20th anniversary... but would be Season 21 by current official numbering.
 * It got even more complicated when Samurai was extended to two seasons, the second part as Super Samurai. Beyond the possibility that Gokaiger might miss the anniversary, nobody's sure whether to consider Super Samurai as Season 18b, 19, or 20.
 * Long before that, there was Mighty Morphin Alien Rangers, a transitional period between the third season of Mighty Morphin Power Rangers and Power Rangers Zeo. It's commonly considered Season 3b, being too short to be a season in itself and sharing enough with Mighty Morphin' S3 to be lumped in with it.
 * Super Sentai itself also underwent a similar situation. The first two Sentai shows, Himitsu Sentai Goranger and JAKQ Dengekitai, which Toei co-produced with Ishinomori Production, were not counted among the later Super Sentai shows that Toei produced independently starting with Battle Fever J. As a result, Choujuu Sentai Liveman was produced as the tenth anniversary series and Gosei Sentai Dairanger was originally considered the fifteenth series. Somewhere along the lines Goranger and J.A.K.Q. were retroactively added to the franchise's count, making the earlier anniversary shows before their inclusion seemed odd in retrospect.
 * The Kamen Rider franchise has a similar issue, not with the number of shows, but with the official number of main Riders:
 * The first Kamen Rider series had two main Riders: Rider 1 and Rider 2 (the latter started off as a temporary substitute of the former).
 * Kamen Rider V 3 (the second Rider series, which centered around the titular third Rider) featured a secondary character named Riderman, a villain who underwent a Heel Face Turn. Despite the fact that Riderman was not a main character, he shows up in many of the subsequent crossover movies and specials as the fourth main Rider.
 * Kamen Rider Stronger, the seventh Rider, had a female sidekick named Tackle. Unlike Riderman though, she is not counted as an official Rider.
 * Kamen Rider Black and Kamen Rider Black RX were originally counted as one Rider, since they were different alter-egos of the same character (RX being an upgraded form of the original Black). Thus, RX was counted as the eleventh Rider when the older Riders guest-starred in his show. However, Black and RX have been counted as separate characters ever since RX guest-starred in a two-part episode of Kamen Rider Decade where he teamed up with an alternate universe version of himself who retained his original Black form.
 * All of the Rider shows from Kamen Rider Agito and onward had numerous secondary Riders (including movie-exclusive characters) in addition to the titular protagonists. Kamen Rider Decade established the official number of Riders by counting all of the Riders from Rider 1 to J (with Black and RX being established as separate individuals at this point) and counting only the titular Riders from Kamen Rider Kuuga and onward.
 * The official count leaves out the alternate versions of the first three Riders who were in the reboot movies (The First and The Next), as well as Kamen Rider G (a Rider created for a one-off parody).

Music

 * The Traveling Wilburys named their first album Traveling Wilburys Vol. 1, and their second album Traveling Wilburys Vol. 3— eit her as a joke, or to reference their involvement in the charity album Nobody's Child: Romanian Angel Appeal, which featured a track by the Wilburys, as well as two George Harrison songs, as a supposed Vol. 2.
 * Chickenfoot's second album is Chickenfoot III to spoof Sophomore Slump.

Software

 * Windows 7. In the software numbering, it's 6.1. Going on major design changes, it is the seventh iteration (NT/95/98/ME being the fourth, 2000/XP being the fifth, Vista being the sixth). But as far as major releases go, it's the ninth (NT/95, 98, 2000/ME, etc).

Tabletop Games

 * The various Dungeons & Dragons editions are titled Dungeons and Dragons, The Dungeons and Dragons Basic Set, Advanced Dungeons and Dragons, Basic Dungeons and Dragons, Advanced Dungeons and Dragons, Second Edition, Dungeons and Dragons, 3rd Edition, Dungeons and Dragons, Revised Edition, Dungeons and Dragons, 4th Edition, and Dungeons and Dragons Essentials. Which means that "4th Edition" is actually Version 8. Whether Essentials counts as Version 9 or is simply Version 8 renamed is the subject of some debate among the game's fans.

Web Original

 * Discussed in The Angry Video Game Nerd's video "Chronologically Confused about Movie and Video Game Sequels"

Video Games

 * Silent Hill had four games, a prequel (Zero), an oddly named sequel (Homecoming), and a remake of the first (Shattered Memories). The next, Downpour, was slated to be released as 8 until someone realised the problem.
 * Super Metroid and Metroid Fusion was called "Metroid 3" and "Metroid 4" respectively in their opening titles. Metroid Prime came out the same time as Fusion, and was intended to be just an aside between the original and Return of Samus. But then the Prime series was a runaway success, creating 2 sequels and 2 spinoff titles. And then, there's Metroid: Other M, an interquel between Super and Fusion, that came out in 2010. This may prompt Nintendo to drop the classic system of numbering if the 2D sequel to Fusion ever comes out.
 * Street Fighter, which was released in the order I-II-Alpha-III-IV. Chronologically, it's I-Alpha-II-IV-III.
 * Then there's the Street Fighter EX series, a polygonal offshot of Street Fighter II. In terms of plot, the original game was intended to be a side-story set during the events of II, but since the series was developed by Arika (who own the rights to the new characters introduced in the series), it evolved into its own continuity instead.
 * Final Fantasy IV was originally released in America as II, and Final Fantasy VI as III. Synchronising the sequels as of VII confused Americans briefly, but the numbering has caught on.
 * The Virtual Console releases of Final Fantasy IV and VI in America, being straight emulation of the SNES versions, kept the earlier Americanized numbering, despite the fact that there were already ports of those game for the PlayStation and Game Boy Advance that restored the original numbering.
 * Then you had Final Fantasy X-2 (and later, Final Fantasy XIII-2), which muddied things further.
 * StarCraft was initially released on 31 March 1998. By 2009 the franchise included various novels, add - ons, etc., as well as a major Expansion Pack, Brood War. When Starcraft II came out in 2010, there was a noticable Double Take by some fans at the fact that it was "only" the first sequel.
 * Similarly, the Homeworld franchise consists of three major installments, Homeworld (1999), Homeworld: Cataclysm (2000), and Homeworld 2 (2003)
 * Call of Duty had it's fourth game named Call Of Duty 4: Modern Warfare. Its sequels are named Modern Warfare 2 and Modern Warfare 3. Then there's Call of Duty Black Ops, which is arguably a spin-off from Modern Warfare, but isn't given the Modern Warfare tagline.
 * The Gorky series began with Gorky 17 (also known as Odium), then Gorky Zero and then ''Gorky 02'.
 * Although it does not bear a numbered title, the 2011 version of Mortal Kombat is officially considered to be the ninth game in the series by the developers (excluding updated versions like Ultimate Mortal Kombat 3). The crossover game Mortal Kombat vs. DC Universe is the eighth game.
 * The Legacy of Kain series started out with Blood Omen: Legacy of Kain. The sequel shifted the subtitle to the forefront and was titled Legacy of Kain: Soul Reaver. This was followed by Legacy of Kain: Soul Reaver 2 and then by Legacy of Kain: Blood Omen 2, the reasoning being that they were subseries, the Blood Omen games featuring Kain as protagonist and the Soul Reaver games following Raziel. The developers finally did away with the "numbered subseries" idea when they released the fifth and currently final game titled Legacy of Kain: Defiance.
 * The Ace Combat series contains six numbered games (of which 1 and 2 weren't originally part of the main continuity, and 3 is set after 04, 5, and 6) and five oddly-numbered ones: Advance (prequel to 3), Zero (prequel to the entire series and, more specifically, 5), X (released before 6 but set after it), Xi (interquel to X), and X2 (which was eventually stripped of the "number" because it had nothing to do with X except the platform). Thankfully, Namco Stopped Numbering Sequels at Ace Combat: Assault Horizon.
 * An Older Than the NES example of this trope is Atari's Sprint series of arcade games. Sprint 2 started the series in 1976, followed by Sprint 4, Sprint 8, Sprint One (switching from Arabic numerals to words), Super Sprint, Championship Sprint, and finally Badlands (which is Sprint in a post-apocalypse setting). The confusing thing is that the numbers in the first four Sprint titles do not indicate the game's order in the series. It actually indicates how many human players can race at the same time. So Sprint One got its name for being a one player game, even though it was the fourth in the series chronologically.
 * The first game in the Gex trilogy is simply called Gex, and the last one is called Gex 3: Deep Cover Gecko, but the second game is called Gex: Enter the Gecko, no "2" involved.
 * The Battlefield series has this. Battlefield 2 was actually the third installment, for instance (which makes some sense; Battlefield Vietnam wasn't as well-received as 1942) . Battlefield 3, on the other hand, is at the very least the eleventh game in the series.
 * Resident Evil Code: Veronica was designed and written to be the true sequel to Resident Evil 2, but since it was originally released as a Dreamcast-exclusive at a the time Resident Evil series was still tied to the PlayStation, it was treated as a side-game rather than as a numbered sequel. As result, the number "3" was instead given to Resident Evil 3: Nemesis, a side-game developed on the second game's engine. The next two games were a remake of the first game and a prequel titled Resident Evil Zero (both originally released for the Game Cube) and as a result, Resident Evil 4 is actually the seventh game in the mainline series.
 * The arcade version of Contra was followed by a solo sequel titled Super Contra and both games were eventually remade for the NES (the latter being shortened to Super C). The NES games were followed by an SNES sequel titled Contra III: The Alien Wars and all subsequent sequels were left unnumbered until the release of Contra 4 for the Nintendo DS, which was actually the eleventh (non-port) installment in the series and an interquel set between Contra III and Contra Hard Corps. To add further confusion, Contra III was actually the fourth game, since there was a Game Boy game titled Operation C released between Super C and Contra III (which was likely not counted at the time since it was a portable release).
 * Note that this was never much of an issue in Japan, where none of the Contra sequels were numbered: Contra III was originally titled Contra Spirits and when Contra 4 was localized there, it was retitled Contra: Dual Spirits.
 * The original Shinobi was released for the arcades in 1987, which was followed by two separately-developed sequels in 1989: an arcade sequel titled Shadow Dancer, which featured the same play mechanics as the first game (but with the addition of a canine companion); and a sequel for the Sega Genesis titled The Revenge of Shinobi, which featured completely different play mechanics from the arcade games (with a few elements borrowed from the Master System version of the first game, such as a health gauge and multiple ninjutsu spells). The Genesis later received two additional Shinobi games in the forms of Shadow Dancer: The Secret of Shinobi in 1991 (a remake of the arcade Shadow Dancer with the same play mechanics, but with different stages) and Shinobi III: Return of the Ninja Master in 1993 (the true sequel to The Revenge of Shinobi). To add further confusion, there was also a Master System-exclusive sequel released in Europe and Brazil titled The Cyber Shinobi: Shinobi Part II.
 * The Wonder Boy sequels consists of Wonder Boy in Monster Land (aka Super Wonder Boy: Monster World), Wonder Boy III: Monster Lair, Wonder Boy III the Dragons Trap (aka Monster World II), Wonder Boy in Monster World (aka Wonder Boy V: Monster World III) and Monster World IV. Most of these alternate titles were the result of the games being retitled in different regions, but even in Japan the console ports of the first two arcade games were released under different names as well (the original Wonder Boy was retitled Super Wonder Boy on the Mark III and Monster World was the title of the Mark III port of Monster Land). Monster Lair (the third arcade game) and Dragon's Trap (a Master System game) were developed and released almost at the same time, resulting in two different games being titled Wonder Boy III (although Dragon's Trap was released exclusively for the overseas market at first and only made it to Japan a bit later via a Game Gear port released shortly after the game's own sequel Monster World III).
 * Sonic the Hedgehog is starting to get like this. Officially, it goes Sonic The Hedgehog (released '91), Sonic 2 ('92), Sonic 3 ('94), Sonic & Knuckles ('94) which is a direct continuation of Sonic 3, Sonic CD ('93), Sonic 4 Ep. I (2010), and then Sonic 4 Ep. II (due 2012).
 * There's also the Dreamcast games Sonic Adventure (ported to the GameCube as Sonic Adventure DX), followed by Sonic Adventure 2 (ported to the GameCube as Sonic Adventure 2: Battle, not to be confused with the Game Boy Advance game Sonic Battle). Storylines from this series continue into the multiplatform releases of Sonic Heroes, and then further into Shadow the Hedgehog.
 * There's also Sonic 3D Blast, which was renamed Sonic 3D: Flickies Island in Europe, which has no direct connection to any other game, not in the least Sonic Blast for the Game Gear.
 * Assassin's Creed seems to be heading down this road. The first game was simply Assassin's Creed, the sequel was Assassin's Creed II, followed by Assassin's Creed: Brotherhood and Assassin's Creed: Revelations. Due out in 2012 is Assassin's Creed III, which is the fifth game. And that's not counting the spinoffs...
 * The arcade sequels to Out Run consists of Turbo Out Run (1989), Out Runners (1993), Out Run 2 (2003), Out Run 2 SP (2004) and Out Run 2 SP SDX (2006).
 * Double Dragon V: The Shadow Falls is a fighting game spin-off of Double Dragon that featured character from the Double Dragon animated series. While there was no actual game titled "Double Dragon IV", Double Dragon V seems to be counting the SNES-exclusive Super Double Dragon (aka Return of Double Dragon) as the fourth game in the main series. Whether anyone considers Double Dragon V to be a true Double Dragon sequel is a whole 'nother debate (since it's a game of a completely different genre developed by Leland Interactive without Technos' involvement and with little resemblance to the previous games).
 * Bubble Bobble (originally released for the arcades in 1986) was followed by numerous sequels such as Rainbow Islands: The Story of Bubble Bobble 2 (a 1987 arcade sequel that played nothing like the first game), Parasol Stars: The Story of Bubble Bobble III (a TurboGrafx-16 sequel to Rainbow Islands released in 1991), Bubble Bobble Part 2 (an NES sequel to the original game released in 1993), Bubble Symphony (the third arcade game in the series, released in 1994 and also known as Bubble Bobble II in North America) and Bubble Memories: The Story of Bubble Bobble III (the last game in the main series, released for the arcades in 1996)
 * Tatakae Genshijin 3, the third game in the series and the second one to star Joe & Mac (the protagonists of the original game) is known as Joe & Mac 2 in the US and Joe & Mac 3 in Europe (note that there was a Joe & Mac Returns released around the same time, but it was an arcade game, whereas Joe & Mac 2/3 is an SNES game). For those curious, Tatakae Genshijin 2 was Congo's Caper.
 * Grand Theft Auto... hoo, boy. It goes Grand Theft Auto, Grand Theft Auto: London 1969, Grand Theft Auto: London 1961, (the London games actually being mission packs), Grand Theft Auto 2, GTA III, GTA: Vice City, GTA: San Andreas, GTA IV, GTA IV: The Lost and Damned, GTA: The Ballad of Gay Tony (the latter two being DLC), and then GTA V. If you include the portable games and Stories games, Grand Theft Auto V is the 15th overall game in the series.