Dungeons of the Unforgiven (Dos):
The monster map for this game could be deleted causing the monsters to reset their positions on the map. If you knew where a monster that was really beneficial to be killed was located, you could kill it, save/quit, delete the monster map, and then load your game back up.
Ultima: The False Prophet (SNES version):
There's a glitch where you can get tons of free items. The gist of it is that you have to have enough items in your storage sack in your inventory so that you can scroll down past the bottom of your normal inventory in the sack (which takes something like 16 items if you have nothing else in your inventory). When you exit the sack you will be below your normal inventory. From there you can scroll up to get to your inventory, or scroll down to find hundreds of different items including armor, weapons, and key items from the game. You can pick those items up and move them to your main inventory then and have them for your own use. This glitch does occasionally kill all your party members without sending you back to lord british, which means you have to load your last save, so always save before use. I've never seen this posted anywhere before I posted it on gamefaqs a few months back, but I've known about this for around 15 years.