THM350 (1970 Pontiac LeMans Sport) - My daily driver failed with 93,000 miles on it. Lost reverse and it was sudden, no warning. Although I was a kid at the time I can say confidently I didn't abuse the transmission in my time as the owner, I do not recall ever "rocking" the car to get out of snow etc. I did not take the transmission apart, I traded it as a core for a rebuilt (something you would never do today, as the born with VIN stamped transmission now matters on old cars, back then it meant nothing) so I do not know what happened.
THM700R4 (1987 Chevrolet Silverado 2wd) - 30,000 kms (18,000 miles) on the vehicle, never abused, never towed anything, I carried my dirt bike in it on weekends that was it. We were coming home through the mountains and the traffic was fast that day so I was clipping along at 125 kph (about 78 mph), there was a bit of hunting going on between OD and drive but nothing I would deem excessive. I always pulled the shifter to "D" if the hunting was excessive (and still do even though powertrain management is much better now). The first symptom was the truck stopped shifting into OD towards the end of the trip (I was 40 miles from home). I wasn't too concerned yet, maybe I was pushing a wind etc. I stopped at the first set of lights on the outskirts of the city and then I knew it was bad (smoke, smell boiling up from underneath). Light turned green and I limped it home, it was no longer shifting at all, just 1st gear. No root cause on this one either. I had the shop rebuild the VIN stamped transmission, they used a Corvette servo and a shift kit etc. all the tricks for a 700R4. I installed a large aftermarket cooler as well. No issues for the rest of the life with the truck. However I was very disappointed in everything about this... age of the truck, GM's response, crap transmission, cost to me etc. Subsequent trucks I have owned with the 4LE60 have been way better in the powertrain department. My father also had a 1987 4wd at the same time as my 1987 but his truck had the 3 speed THM400 which was a better choice for sure just no OD. I have has many THM400's in my old muscle cars and that was a bulletproof transmission.