The Ooze. My memory on this is fuzzy but on genetics lab part 2, there is a room you can enter that has a checkpoint. If you enter the room then you’re locked inside and if you collect the checkpoint and die, you will respawn back into the room and your only option is to lose all your lives or reset the game. I remember getting really pissed off finding this when I was a kid because I spent days trying to beat the game and I had a really good run up until that moment.