This bug is pretty huge (probably the biggest in a list of issues with multiplayer).  In Grand Theft Auto IV, the co-op mode called Bomb the Base II is basically unplayable (Xbox 360 version, not sure about PS3). 

If you have more than one person playing (say, you and a friend), each of you get a limited number of lives.  If one of you dies too many times, the mission fails (if you are playing with 3 people, 2 people need to run out of lives for the mission to fail).  This in itself is fine, however if the other player still has lives left while the mission fails, they will be stuck staring at a screen waiting for the lobby to load, eternally. 

The player that lost all their lives is properly loaded back into the lobby, however the player with some lives remaining is stuck staring at the boat.  You can't move, and you can't choose any menu options other than quit.  Nice one, Rockstar.  So basically with this mode you have two choices:  win, or re-load the single player so that you can re-load the multiplayer to give it another go.  WTF????  So much for QAing this mode, I guess...

Rockstar, please issue a patch to fix this mode so all players properly return to the lobby.  Thanks.