Another co-op bug:
When a saved game is loaded and a second player invited to the game, if the second player is controlling a character that has hitpoint modifying equipment on then his or her character will appear to be missing that much health. The host, however, seems to see the character at full health.
Example: Sue is hosting a game for her and Bob. Upon reloading a saved game, Sue will see Bob's character at full health, but Bob will see his character missing 30 points of health because his character happens to be wearing a ring that gives a 30 HP boost.
This is a bug that existed in the original D:OS and was eventually fixed. Also, I'm referring to a co-op game played over the internet, not a couch co-op session.