This often happens after a bike has suffered a blow to the right side, which bends the derailleur hanger and makes shifting into the spokes possible. This probably happens most often when the wheels are removed and the bike is put int he trunk of a car. It takes suprisingly little force to bend a derailleur hanger - they are usaually designed to be the weak link so when trouble brews it is not the derailleur or frame that fails, just the cheap and easily replaceable hanger.
Having a slightly bent hanger will make the shifting unreliable as well as able to smash into the spokes.