I randomly joined people's l4d2 games in my live stream and I pretended to be a bot by copying the profile picture and name of bots in l4d2. I then started speed running to try to surprise randoms. This was my favorite reaction from the whole stream. This speed run route has been patched in the Last Stand update.
Update: I played with my friends and tried to show that this route was patched, but it's possible to do again I think.