C&C Generals had a thing where if you had multiple clients with the same CD key in a multiplayer game everything would be fine for a while, but 10 minutes into the game all the buildings of the people who duplciate keys would explode.

This happened a few times at LAN parties to my friends, some of whome gave each other the game by copying the install directory across. Took us a while to work out what the hell was happening.