Ok, I got three .mp4 files that iTunes will not add to my library. Its happened to me on .mp3's too, but I've been able to re-download them from different sources.
I have a whole TV Season, all the files were added except three. I've tried dragging, add file, add folder, all that blah.
They are fully downloaded (hash check passed), they're all the same files theres nothing different that I can see.
What the hell?