I've been having a lot of problems with this game.
Halo 1: Quit in the midst of Two Betrayals to go to class. When I got back and hit resume, it bumped me to the start of Keyes. Also,,the Anniversary video failed to load in the first mission.
Halo 2: Didn't start unlocking "mission complete" achievements till I completed the first Arbiter mission, and only unlocked that one. Also, despite subtitles being turned on, no subtitles are shown in-game, and during cutscenes, the subtitles trail behind the video.
Still haven't been able to connect to matchmaking.
I'm totally willing to go back and redo the missions that the achievements didn't trigger for (it's Halo, after all, and I love it enough that playing through the same missions isn't a big deal). I just hope they fix this stuff soon, that the achievements are not somehow impossible to unlock now, and that I can satiate my Halo 2 multiplayer hunger in the very near future.