Took my car in for oil/filter change and in some odd circumstance, the young mechanic managed to drain 1 liter of transmission oil out of my car (which I didn't know at the time). After driving for 10 minutes (~5 km), I noticed my reverse gear wasn't shifting properly. After ~30 minutes of driving, my car was smoking from the exhaust pipe (turns out he had overfilled my engine oil too!!).
Took the car back in to complain the mechanic supervisor claims my transmission is still fine and all I need to do is refill the lost fluid. Another independent mechanic later tells me my transmission line is likely fried if I had experienced what I did, and those guys should be responsible to replace everything.
Since fluids have been topped up, I have no proof of who is right. So my question is, if I only drove for maximum 50 km after my transmission fluid was drained and engine oil overfilled, would that be enough to cause serious damage to the car?