"Dooring" or "The Door Prize" is what happens when an occupant of a motor vehicle opens their door in front of an oncoming cyclist. It is apparently the most common cause of injuries to cyclists in Toronto. There are several known cases of cyclists being knocked by a door under an oncoming vehicle. In at least one Toronto case, the result was fatal for the cyclist. In another, the cyclist had to undergo months of very painful rehabilitation after being run over from the hips down by a large truck.
Toronto taxis have recently implemented a program of stickering the rear doors of cabs with a "watch for bikes" message. This is a great, practical step in cutting down on dooring injuries.