Originally Posted by
cale
If the OP's derailleur fell off just because he/she shifted in to the big ring suggests that the dropout is badly fatigued. Because wear is typically balanced, I'd also regard the non-drive-side drop out with similar suspicion. OP, I think that old Trek frame is trying to tell you something.
Steel only becomes fatigued when bent past its yield point. While dropping the derailleur into the spokes can bend the drive side dropout in this manner, there's nothing on the non-drive side to do the same, so the non-drive side dropout will be fine.