The Sims - Did You Know Gaming? Feat. Brutalmoose
During the first mission, Welcome To Mars, if the player kills Dan Mason with the sledgehammer, the game will have a unique game over screen, saying "WTF? You killed your brother!!!".
Contributed by DeerBoarDude
Once the Geo Mod 2 engine was fully implemented, all the game's buildings fell down.

The developers had to go back and make sure their buildings were built with real-life architectural standards in mind so that the buildings would support themselves.
Contributed by IkiFoo