Hit a pedestrian. Get hit by a car pulling out of a driveway. Hit another bike. Hit/be hit by a tree. Same deal as a road really, only people don't expect you on the sidewalk.

Sidewalks are a bad place to ride. No line of sight if you're on a bike. You may have unexpected 6" (15 cm) jumps in pavement height. There may be low hanging tree branches, or obstacles like kids' toys or a store's sidewalk display. If I've gotta be on a sidewalk, I walk my bike. Then drivers understand what I'm doing. Other pedestrians understand, *and* I'm moving at a speed sidewalks are designed for.