ToME: the Tales of Maj'Eyal

Enter a new zone (not an already generated one) -> use a time travel talent and go back in time with it (any class except Oozemancer can get See the Threads!) -> leave the zone -> the zone didn't get saved, enter it again and it will be generated anew. You can use this to redo easy levels or hidden vaults an unlimited number of times. You can also use this to softlock yourself by generating a required unique enemy and then re-generating the zone without killing/looting that enemy.

Expanding on this, time travel can be used in this manner to make level saves fail in general, so you can also use it to keep an old save of a level around, letting you make copies of items and actors and anything else on that level. A good fix to everything would be to remove time travel from the game.