Somewhere up there was a discussion of carbon fiber vs. Aluminum...
Aluminum has a tendency to bend when it is highly stressed, which leads to small cracks forming and eventual complete failure. Pinching of the bar at the stem is a good possibility, since it really does take a lot of work to bend an AL bar, even a cheap one.
Carbon fiber is a different animal. It's much more resilient than AL so a well-built bar will handle more low-amplitude stress than AL without deforming. The only thing is that if carbon fiber is stressed to its VERY HIGH breaking point, it will fail catastrophically, i.e. explode.
I used a Race Face carbon bar on my mtn bike back when I weighed about 220#, and never had a problem with it. The key is making sure that you're buying from a good carbon fiber manufacturer.