the only trans failure I've never been able to explain was when my 440-t4 died.
the car was driving great.. no slipage. no signs at all anything was wrong..
parked it at the grocery store.. came back out.. nothing.. no drive, no reverse..
if I reved the engine to red line in drive, I could just get the car to crawl about 1 or 2 mph.
the shop never did tell me what failed.