Phobia on Youtube shows off a ton of glitches in The Legend of Zelda: A Link to the Past. Right off the bat they mention that, using these glitches, you can basically just win the game at any time by warping inside a cave wall, finding Ganon’s room in the big connected underworld map, then going one screen up from there, but then they go back and demonstrate not only how to do that, but also how to do a ton of other things.
Some interesting facts revealed, besides that all the dungeons and caves are part of a single map, is that the caves count as their own dungeon, the number of keys there is set to -1, a sentinel value used to hide the key display, but which also functions as 255 keys if you ever encounter a locked door, and that all the Big Keys are the same item, just tracked separately for each dungeon. Meaning, if you can get to a dungeon room with the game thinking it’s a cave, you essentially have infinite keys, and finding a Big Key lets you unlock all the big chests while there.
There’s lots of other little details presented in an accessible manner as well, so if this kind of thing is interesting to you, as it is to me, there’s 35 minutes of it there for you on a plate.
Glitches you can do in Zelda: A Link to the Past (Phobia on Youtube, 45 minutes)