Truncated Theme Tune

A show's Theme Tune is the way it gets the attention, and hopefully is good enough to get people Earwormed and singing it to get others watching the show.

Once a show has gone into syndication or jumped channels, though, the Theme Tune is one of the first casualties. It will get edited, shortened, or otherwise truncated so as to leave room for more commercials. Occasionally, if the show is on the air on its original channel more than a few years, they'll maim the theme tune for more commercial space while it's still in its original airing. Also, the lyrics of the theme song, where present, may be changed a little.

The title sequence and opening credits, since they often appear simultaneously with the playing of the theme tune are often collateral damage. Shows dating from The Eighties and The Nineties are the first to display this trope; older shows' Theme Tunes (Gilligan's Island and I Love Lucy for example) tend to be so iconic that they don't usually go under the editing knife.

Related to Credits Pushback as the end credits usually scroll in the background while the next show or ad for other stuff on the channel plays instead. It comes back full size just in time to hear the last note or two of the closing credits' song and see the studio's Vanity Plate.

This is basically a greed-invoked version of the Second Verse Curse.

Anime and Manga

 * When shown on Adult Swim, Fullmetal Alchemist and Fullmetal Alchemist: Brotherhood's opening and ending themes are shortened significantly when compared to the show's original subtitled version on FUNimation's website.
 * Adult Swim has done this with most of their anime openings and ending, with the exceptions of the openings of Bleach and Code Geass. Most of them get shortened.
 * The Big O. When it first appeared on Adult Swim it had full opening credits and theme tune, as seen here. When it was run later the opening credits were cut down to a couple of seconds and a brief musical phrase. This is a particularly egregious example, as the opening credits for this show were Homages to Flash Gordon and Space: 1999 respectively, and they were lost when the theme music was chopped.

Live Action TV

 * Happy Days: when the show dropped off ABC and began showing up on syndicated TV, the song was shortened and the lyrics changed.
 * Cheers
 * Two and A Half Men: To the point where Neil Patrick Harris made a joke about it while MC'ing the Emmies. He played the show's original theme song. "Men, men, men, men, manly men, men men!" Then he played the shortened version of the song: "Men!" And suggested that the show's theme song in its next season would simply be "Meh--"
 * Superman, the Scrubs theme, is a casualty of this. The full version is 3:40 and goes from already being fairly short at fifteen or so seconds to a mere echo of the theme at about a second. Fortunately, that doesn't last long.
 * Wings (from the creators of Cheers) used to have a title sequence with a tune by Chopin, but that later got (ahem) chopped.
 * Blood Ties has a full theme song and opening sequence, but (at least on Lifetime Real Women where it's currently being rerun), the theme is cut down to a title card and brief musical clip, and the credits are all shown during the next scene.
 * In syndicated reruns of Soap both the opening and closing credits were cut off early. In the opening it'd fade out just after the announcer says "and this is...Soap," cutting off the visual joke; and the end credits are faded out either right after the list of actors, or occasionally even before them. (Neither of these are going on right now in the reruns shown on Antenna TV.)
 * Syfy is notorious for this.
 * Eureka used to have a theme song that lasted about forty-five seconds, and showed Carter walking through town seeing the Mad Science applications to mundane tasks. Now it's five seconds long and has just the Title Card and the names of who created it.
 * Warehouse 13 also has shortened opening credits and theme tune.
 * For Season 9 of Stargate SG 1 and Season 2 of Stargate Atlantis, the intro credits were cut to a 10-second clip, followed by the credits rolling over the next scene. After the fans rioted, the full intros were replaced halfway through the season. However, the next spinoff, Stargate Universe, had only a brief title card and no full theme song.
 * Smallville averted the trope for its entire run, using the same, uncut Theme Tune.
 * Fraggle Rock had a much longer theme song and opening credits sequence (in which the titular Fraggles got to introduce themselves by name) than the one currently showing on The Hub.
 * The US version of The Office features this in syndication, truncating most of the theme song beyond the first eight bars and the final chord.
 * HBO's Curb Your Enthusiasm has had its theme song reduced to a single phrase.
 * Charmed's theme tune is cut down to a 10-second clip with a flash of each main cast member's name for its reruns on TNT.
 * TBS goes back and forth between airing the full Friends theme including the verse, and a version with just the chorus.
 * They do the same thing for King of Queens.
 * The Outer Limits had its title sequence shortened twice during the original series' run on ABC.
 * When The Fresh Prince of Bel-Air is shown on TBS, it has just the background audio to the theme song played over a sped up version of the opening.
 * The Golden Girls often has the 'And if you threw a party...' part cut out when it airs on Lifetime, though not every single time. It's easy to tell the difference right away by whether or not the shot of a plane landing is the opening shot of the theme or not.

Western Animation

 * Ben 10 is the only show from the franchise that has a Theme Tune with lyrics, so it's particularly noticeable when the second verse of the song is omitted in the Boomerang airings. The Theme Tunes for Alien Force and Ultimate Alien are instrumental and were designed to be very short.
 * The Spectacular Spider-Man lost a verse of its theme song when it moved from The CW to Disney XD.
 * One infamous example is when Animaniacs was put on syndication for Nickelodeon. The lyrics for one of the opening gags "Shirley Mclaney" was shortented to "Nick-a-laney" in order to fit with the blatant "It's on Nickelodeon now" message (complete with the Nickelodeon logo obnoxiously included in nearly every shot during the opening credits). Also, other parts were cut down even further, and one part was even sped up slightly to keep it all in the same key. More details here.
 * The theme tune and opening credits to The Simpsons exists in its full form, but there are several diferent cuts to adjust for show length, and extended couch gags to fill time.
 * An interesting variation: sometimes the opening theme is sped up to save time rather than truncated. This happened noticeably, for example, in Justice League Unlimited.
 * Danny Phantom has at least two other verses of his Theme Tune, the musical riff usually played over the title card, and the bridge played at the end credits right before the Billionfold logo. Both were cut before the show went to production to leave the opening credits as we know them today(at least in the US - this may differ for other countries). See the variant here.
 * In the Comedy Central revival episodes of Futurama, several seconds of music toward the end of the theme have been cut out. The original episodes remain unchanged.
 * The theme to the 2011 reboot of ThunderCats is a rearranged version of the 1985 Thundercats Theme Tune, now made lyricless and consisting solely of the original's Fanfare, its signature " Thunder thunder thunder Thundercats!" Epic Riff, and a finishing roar. All ten seconds are performed by a full orchestra while the assonances are called out by an Ethereal Choir. This makes for a surprisingly effective aural Adaptation Distillation.
 * When several episodes are strung together for a block or marathon, Nickelodeon will usually lop off the entire title sequence for Avatar: The Last Airbender.
 * Around its fifth season, Fosters Home for Imaginary Friends had its theme song shortened and sped up. The original theme came back for season six.
 * KaBlam!! had its theme song shortened for season three onwards.
 * In syndication, the closing theme to The New Scooby Doo Movies would sometimes cut from the gang yelling "Hey Scooby!" to the final chord, omitting Shaggy's "Where are you?" and Scooby's "Over here!", while the Hanna-Barbera "Swirling Star" logo appeared.
 * Ed, Edd 'n' Eddy, when it was put back on Cartoon Network in 2012.
 * Young Justice came back from its second season hiatus with a 3 second riff and its title card, but this was at least in part to make room for the DC Nation shorts.
 * Green Lantern: The Animated Series also has shortened the theme tune down to 3 seconds and its title card, also to make room for the DC Nation shorts.
 * Midway through its run on Fox Kids, Beetlejuice had its opening shaved by two-thirds.
 * Ultimate Spider Man has a full theme song but because of Marvel Mash-Up, Marvel Master Class and the Fury Files portions of Marvel Universe on Disney XD, it never once got to play as the opening of the show.