Just got my commuter bike back from the shop, after they replaced the broken frame. However, I couldn't use the middle chainring from a dead stop. A loud bang, and the damn chain dropped down to the little chainring. This has the effect of putting a serious dent in my ability to cross a street from a dead stop.
Try as they might, the LBS's mech's couldn't reproduce the problem. Yet, with them watching, I duplicated it over and over again. I was told that it was just a new chainring, and the sharp edges that needed to wear in. I've never had that problem before, no matter how big I was (282lbs/127.91kg right now).
Well, it's been a week, and I was not happy with this "wear in" solution. So, I went back. I demonstrated it again, this time to a different mechanic, while I was in the store. The manager just about jumped out of his skin when that loud bang went off.
The solution? Turns out that the guy who built up the bike put the middle chainring on backwards. Apparently, the chain (which, like the rings, and the cassette is brand new) has worn already from the abuse. They are fixing the issue at their cost.