In one accident, some of the parts under the engine came off jamming the engine and the cars behind telescoped into each other and the engine.
As late as December 2010 (several years later) the Big Thunder Mountain engines are missing their middle large wheels. Orignally there were two large wheels on each side, now there is just one. Those middle wheels are for decoration only and apparently it was those wheels that fell off. All of them on all the trains were then intentionally removed.