One-Time Dungeon

In video games with wide open worlds built on revisiting the same locations over and over, there are certain places that you can only enter once and never again. Sometimes this is because the place no longer exists thanks to being a Doomed Hometown or a Collapsing Lair. Sometimes it's because the plot keeps you out of it. Whatever the reason, these are places that can never be returned to.

The items inside the dungeon are frequently Lost Forever and there are often times that you can only fill up your Monster Compendium by grinding there.

Note: In order to qualify a game must be generally be open to Back Tracking and not just have a linear advancement through it.

Action

 * In the first Xbox Ninja Gaiden, you can't go back to the Ninja fortress of chapter 1 nor in chapter 3's airship since it crashes.

Action Adventure

 * The Legend of Zelda series generally avert this, but it has some examples:
 * The Legend of Zelda: Phantom Hourglass has the fourth dungeon, the Ghost Ship. It has a The Dev Team Thinks of Everything moment: if you leave without taking its Heart Container, the only item in the dungeon relevant to One Hundred Percent Completion, it is sent to you via mail to avert it being Lost Forever.
 * The Legend of Zelda: Skyward Sword has the final dungeon, the Sky Keep, which disappears once you  It has no Heart Pieces.
 * Zelda II: The Adventure of Link provides an example with the Palaces that become Mountain squares on the overworld map after you beat them, but only after you both defeat the boss and collect the required item, fortunately. The only things that can truly be Lost Forever are some Experience Point bonuses.

Action RPG

 * Okami has the Moon Cave dungeon that cannot be accessed again once your defeat Orochi and leave it. There is also Oni Island, the miniature sections of the Imperial Palace, the Sea Dragon's body and Past Kamiki. This is not frustrating though since these areas will only contain Vendor Trash, but no important secret item.
 * The sequel Okamiden is not so forgiving though. It has the Five-Story Pagoda, the Moon Cave (again), the Underground Ruins, the Ice Cavern... All of these contain unique items that are needed for 100% Completion.

Eastern RPG

 * Final Fantasy XII has several, and in an uncharacteristic act of kindness, the programmers indirectly tell you which ones by giving you a prompt about not being able to leave them until you finish them when you try to save in them:
 * Nalbina fortress which is only there in the prelude due to it being destroyed.
 * Rabanastre Palace during the heist.
 * Nalbina Dungeons which is only accessible during your escape.
 * Several airship dungeons including the Leviathan and Shiva.
 * Draklor Laboratory in Archades.
 * Final Fantasy III has a floating Hyne Castle, which after finishing gets planted into a forest you're not allowed to get back to.
 * Final Fantasy IV has the Tower of Zot which collapses once finished with.
 * In both MOTHER and Earthbound, Magicant vanishes. In the first game, Magicant can be revisited prior to completing a particular subquest; in the latter, it's a true One Time Dungeon with a few special items that can be Lost Forever.
 * Mother 3 uses a Chapter system, but most dungeons and areas in the game can be revisited in at least one later chapter (although the enemy types and layout will change with the plot). The major exceptions are the Attic Dungeon in Chapter 4, which becomes inaccessible once the Chapter ends with its completion, and the Thunder Tower, which is only accessible in Chapter 5 because the story at that point involves infiltrating and destroying it.
 * Final Fantasy VII has several locations that can only be visited once to name of. Sectors 1 and 5 reactors, Shinra Headquarters (won't be revisited again until the raid late in disc 2), Temple of the Ancients, Corel Prison, Gaea's Cliff, Underwater Reactor, and Midgar during the raid.
 * Final Fantasy VIII: There's Dollet Communications Tower, Galbadia D-District Prison, Missile Base, Galbadia Garden (during the attack), Great Salt Lake and Lunatic Pandora (only visited twice) leading to almost all locations inaccessible due to Time Compression. Not to mention the Laguna dream sequences.
 * Final Fantasy X has the Bevelle temple and the Via Purifico. The annoying part is that if you didn't get the treasure in Bevelle Temple, you won't be able to get Anima.
 * The Distortion World in Pokémon Platinum.
 * N's castle in Pokémon Black and White is destroyed and can not be revisited.
 * The dungeons you explore in the future and The Nightmare in Pokémon Mystery Dungeon Explorers of Time/Darkness/Sky
 * The S.S. Anne in Pokémon Red and Blue.
 * In Chrono Trigger, the Black Tyrano fortress in 65,000,000 B.C. is destroyed when Lavos crashes into it. Mt. Woe in 12,000 B.C. is no longer accessible after the chain breaks and it falls into the sea. The Ocean Palace in 12,000 B.C. is no longer accessible after.
 * is also a possibility, depending on
 * In Breath of Fire, Nanai and the dungeon beneath it are destroyed after you leave. Neither Mogu nor Mote's dream dungeons can be re-entered once you complete them. The Goddess' Tower is also destroyed.
 * Mario & Luigi: Superstar Saga has the entire beginning of the game; specifically, Mario's house, Peach's castle (where you fight Bowser), Toadtown Square, the Koopa Crusier, and the small bit in Stardust Fields before the Border Jump minigame.
 * All story dungeons in Hyperdimension Neptunia.

Western RPG

 * The Elder Scrolls IV: Oblivion has the Painted World area and a dream sequence that can't be returned to. Closing an Oblivion gate destroys that instance, although it's possible (and indeed likely) to find an identical map behind another gate, but completing the main quest line permanently closes all the gates, cutting off access to Oblivion entirely.
 * Eamon. The Beginner's Cave (which was on the game's Main Hall disk) could only be gone through once. The game checked when you tried to access it: only a true beginning character was allowed to enter, and once you had gone through it you were no longer a beginner.
 * In Knights of the Old Republic 2, Goto's Yacht cannot be revisited, as it is destroyed after you complete it.
 * In Fallout 3, completists would be advised to grab the collectible Energy Weapons bobblehead during the brief Raven Rock sequence, since once the door shuts behind you, you're never getting back in there.
 * The Sierra Madre DLC of Fallout: New Vegas cannot be visited after it is completed, unlike the Old World Blues, Honest Hearts and Lonesome Road DLC. It's not a problem in the PC version, though, as you can use the noclip console command to find a door that takes you back.
 * Diablo II: The Cow Level can only be played once per difficulty level.