Reviving this old thread to add a data point: Riding my '85 Trek 600 Series through some rough gravel, the right side of the alloy bars failed at the stem. I'm guessing it's just 32 years of fatigue; there is no evidence the bike has ever been crashed or otherwise abused.

I got lucky; I was only going about 12-14 mph due to the rough surface and brought the bike to a stop without crashing. I was able to very gingerly nurse it home about 5 miles.