The first headlight I ever bought had a broken mounting bracket within a couple months. My new one (which is expensive, and may be overkill depending on your situation) seems difficult to break. It's one of the big Sigma brand lights with an external rechargable battery.
I had the back light on my bike break recently. I was about to park my bike, and when I hopped the curb to get on the sidewalk it fell off. Anyone else ever have this happen to them? I can't figure out what would have put enough stress on it to cause it to crack like this.