I have a problem playing campaing Halo 1 ( Co-Op, Legendary ). When we found a secret oracle video, the game send us to halo channel so when the video its done the game send us to the main menu and all progress and chekpoints its gone and we have to start over again and also in the middle of the progress we pause and the game send us automatically to the main menu and we lose everything so again start the chapter from the beginning. We dont know if this going to be happend in Halo 2,3 and 4. PLEASE FIX THIS BUG ASAP…!!! Thank you. 
Same thing happened to me and my brother when we were playing co-op to, we would pause the game for a while and then it would kick us to the main menu. It did the same thing when we would watch the videos when finding the terminals and when it kicked us we would lose all of our level progress. Also when completing levels it would give us the achievements, or at least it did until our achievements stopped unlocking, but it only showed that the lobby leader completed the level. When we went to continue the campaign with the other player as the lobby leader it showed that he didn’t complete any levels but when we signed in the original lobby leader who had been the lobby leader the whole time we were playing only then did it show that we completed the levels. So it’s only giving the lobby leader credit for completing the level not the other player. PLEASE FIX THIS!
They say tomorrow will be other update to fix the campaing bug problem. We hope .!!