Anyone know why this is happening? Has it happened to anyone else?

So I've been playing a lot of Modern Warfare 3 and just recently when I got out of the game, it started to update. Ok whatever, I'll just let it update. It started I think at like 96% and when it was done I tried running it again. It showed the splash image and then an error pops up about missing some file. I don't remember which file it was, I just closed it and verified the integrity cache. Well, it said 125 files failed verification and needed to be reacquired. So I'm going to leave Steam running while I go to work tonight, reacquiring files. No idea why this happened.

I don't use any kinds of cheats, hacks, bots or anything like that. I don't join private servers. I used to join some for infected, but that was a while ago and everything's been working fine since.

I'll update when I come back from work in, I don't know, 8 hours?