Originally Posted by
cyccommute
Given the severity of the bend on the fork, you aren't looking at a "dent" in the downtube but a buckling of the tube itself. Nothing 'hit' the tube to dent it but, rather, the tube tried to fold upon impact ......
This is a nice theory, except that it disregards all the physical evidence. The shape of the dent shows that it's a dent, and not a buckle. Moreover it's precise alignment with the fender brace, stretches credulity as a coincidence. What almost certainly happened is the fork pushed back pushing the wheel into the downtube with the fender brace trapped between them.
In any case, when downtubes buckle from front impacts it's almost always at the bottom directly behind the headtube. The reuslting buckles have a characteristic appearance (clearly a buckle) which this frame doesn't.
As far as safety goes, without any repair at all this frame has a decent chance of outlasting it's owner, and if and when it fails will give generous notice.