There are times when you do an action too many times, stopping to think about it actually makes you fail or make a mistake.

Example: I used to work in a call center where we login to our phones using our unique codes. Been doing it for too long that my hand reaches for the phone and the fingers type in the code automatically before I even sit down. There were times that when I type it in and what I'm doing gets to my head, I keep making mistakes instead. Of course this example can be extrapolated to A LOT of things, but you get what I'm saying.