If your computer or connection lags too much on co-op mode, sometimes you will lose connection and be removed from the game back to the menu. Sometimes though, when the connection is lost, the screen just turns black with the spinning Pentagram at the bottom as a loading screen, but it just stays there. You are stuck. You have to close the game down and restart it.
Labyrinth of Death: After the first safe room, you have to go and hit the control to manually open a large door. It takes the door a few seconds to finally start opening. There is a horde of zombies on the other side waiting. Once it opens, if you are zoomed in with binoculars, your sniper scope, or happen to be right next to the door, you can see the zombie in the middle at the front is standing there saluting for a moment, and then snaps out of it and starts coming after the players with the rest of the horde.