I had a much delayed trip to MGM from AKL once because a car had hit a Disney bus bound for AKL that was pulling out of Blizzard Beach. Apparently they had to dispatch another bus to AKL to pick up those of us who were waiting there. We did the stop at Blizzard Beach, of course we had to wait in traffic around the accident, and the passengers were still on the bus involved at that time. Most of the damage appeared to be to the car, didn't look like there were any injuries.
All the busing around WDW I have done, that's the only time I've seen a bus in an accident. I've seen one broken down on the side of the road before.