Game-breaking glitches

Final Fantasy VI has a few of them. Most famously the Opera House Glitch.

Sorry I hadn't updated this for a while! I couldn't access the internet for two weeks.

Meanwhile I remembered another game where you can get permanently stuck, but in this case instead of a glitch it's an oversight in the level design. There's a rather obscure platformer/dungeon crawler called Equinox for the SNES. In one of the later dungeons if you go too far into it before you've collected everything you need later on to progress, you can't get back to those items if you also missed an optional key. Save there and you're trapped. It's explained better in these videos:





The key in question is at 4:20 in the first video and the gate is at 9:05 in the same video. At 2:25 in the second video there's a one-way room leading to a passage that eventually branches your path to the other side of the same gate at 6:30.