In some missions, I just popped out of the map and into the outside. That happened to me twice today. In the first time, I late joined a game and spawned somewhere outside the map. I tried to jump back in only to keep falling until I die. In the second time, I didn't late join, but the game was laggy and soon I just popped over a part of the tile set and the ceiling was solid. Still, I could shoot below, and the enemies down below could still shoot me when I'm up there.
When that happens, I can't join the other players on the mission. It's a glitch that happened somehow... I like glitching, though. :P
I even took a picture of it: https://fbcdn-sphotos-g-a.akamaihd.net/hphotos-ak-ash3/t1/1656227_223398554528907_2071581481_n.jpg