in l4d1 coop there's an exploit where you can use a jointeam command to add extra bots which will screw up the matchmaking.
To reproduce do the following
- When you join a game immediately type "spectate" in the console
- Choose a character that you know is inhabited by a live player, for example when zoey is being occupied type "jointeam 2 zoey"
You will appear as a dead player about to be rescued from a closet and the bot you were originally supposed to have taken over becomes redundant.
It's annoying if people do that because if you do that, the matchmaking will no longer pull in that extra player if you leave, and free spots will be permanently bugged for that session.