The professor may get stuck on a table and a chair in the ballroom
Wait at the checkpoint for 2 to 3 minutes.
Some players may be dropped upon entering the level.
After collecting the three passcodes, you enter a quicktime event; the co-op partner might not see the enemies needed to be shot.
Complete that section in single-player, and continue the game upon boarding the plane.
Crashes the game during prelude.
Turning down graphics settings temporarily during prelude may improve stability long enough to allow you to progress past that stage.
Occurs if the used resolution is unsupported.