Zelda II: The Adventure of Link

Obviously, a lot of dungeon treasures are vital in order to progress the storyline, but what happens if you forget to obtain them?
The temples normally turn into rubble after you finish them, so what do you do then?

There's an easily repeatable way to activate the glitch.
(1) Do NOT beat the first temple; getting the item is optional (but recommended, as it's very useful).
(2) Get the item from temple 2, 3, or 5, but again, do NOT beat it. For this trick, temples 2 and 5 are easiest.
(3) Jump off the top of the screen in the same temple as (2).
(4) Cast fairy while offscreen.
(5) Push left or right.

You'll now wind up an a warped temple. The graphics are identical to the temple you used to be in (2, 3, or 5), but the layout and enemies will be identical to temple 1. If you beat the boss here (Horsehead) and place the crystal, the game assumes you placed it in temple 1, but assigns the "crystal has been placed" data to temple 2/3/5. Since you also got the item there, temple 2/3/5 will collapse (despite not having a crystal). Meanwhile, temple 1 has a crystal in it (and the item may very well be grabbed), but the temple will not fall. Congratulations, you're no longer able to place all six crystals and therefore no longer able to beat this save! (unless you cheat XD)
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Nagas on a plane! ^_^