So I was changing my ATF fluid at 12K myself, and I use a pump to pump in the fluid through the dipstick. I washed the pump with water beforehand because it had some differential fluid in there before. There were some droplets of water remain in the pump and hose after washing and forgot to dry it up. I'm afraid I might introduce some water into the ATF. Should this be a concern for me? I read on the internet even a drop of water here and there can kill the transmission.

 

The little bit of water that might have gone in is nothing to worry about. Just change out your ATF regularly.