I was playing the 2nd or 3rd co-op mission the other day with a friend (offline), I think we had to defend an engine on a train or something. After we killed everybody then defend the train, there was 2 time we had "checkpoint" displayed on screen. Then we both died. Guess what, we were back at the start of the mission.
Seriously, what's the point of the checkpoints if you're back at the start of the mission each time? Only after having finished a whole section in a mission did we get a real checkpoint.
But other times, they seemed to work correctly. Anyone noticed checkponts acting weird? --- I am the danger. A guy opens his door and gets shot and you think that of me? No. I am the one who knocks!