Who remembers the helicopter crash in the Grand Canyon a month earlier? I personally know one of the couples (Daskal, the one that the wife "survived"), a few people were late to work in the Twin Towers because they had a Shloshim Seuda the night before
I think there is a whole book of miracle stories of why people weren't there.
I agree with Dan I could never forget when I found out, was totally surreal, and I wasn't even in NY.
An uncle of mine was there and he said everyone was just standing and staring, he screamed that everyone should run, and people did, then they collapsed and people that ran were saved. It was cool when I heard the exact same story in a book, but it was someone that was staring and he said he didn't move until someone said run...