Wow. I didn't realize my question would bring out so many haters.
I was mid-shoot when this happened. So it wasn't a mounting error. In testing, if the lens is not COMPLETELY CLICKED IN, it reads F00 and MF (manual focus). I suppose it's possible that the release spring didn't engage properly. But I've tested that 100 times and it seems to be in order.
I was not blaming Canon....yet. After Googling the issue and hearing other people's horror stories, I supposed that I accidentally hit the release button and rotated the lens off the camera while zooming. The purpose of asking this question IN A CANON FORUM was to see if any other CANON USERS experienced this issue. If a ton of other people did, then yes, it would be a design flaw and we would hold Canon accountable. But it looks like only a few people have had it happen, so perhaps it's just how we shoot or carry our cameras.
On that note, after more Googling, I think I have an explanation....it happened to a few other people as well. I use a Black Rapid strap over my left shoulder, so the camera hangs on my right hip. I'm guessing that the release got pushed (perhaps by the top of my belt) as it "bounced" around on my hip during the evening. After that, all it took was one good zoom to the right and I twisted the lens right off. I actually feel a little better now that I have a more-plausible explanation than hitting it with my thumb. I can now look at how I carry my camera so this doesn't happen again.
As for the embarrassment comment. I don't get it. I HAVE dropped a lens before...my 70-200L2.8....and I can assure you, embarrassment is not one of the feelings that rises to the top. It's mostly panic and fear (that you won't get your job done, and that it'll cost you a fortune to fix) and then rage...at yourself for being so stupid.