On our '91 190E 2.3, the transmission failed by not shifting out of 1st. It gave no prior warning, but post repair, shifting is much smoother and more consistant than before - if that is related. I never saw signs that the fluid had overheated, condition looked good, and level was correct.
What the shop found on rebuild is that the fluid passages in the unit were plugged with eroded clutch material. I asked if driving pattern could have affected that, but did not get a definitive answer (we drive stretches of mountain declines where lower gears are used for speed control).