There was a time in college I came face to face with a bear. I knew academically what to do. But I screamed like a 7 year old girl stumbled to the ground and ended up running away. In our words, in spite of knowing what I should do, I was completely out of my mind. I was on full auto pilot. I completely understand what it means to be e optionally hijacked. I don't remember anything of what happened between the screaming and the running, and being in my car, seriously... Listening to a cassette tape of Disney songs my wife had from her work at the Disney stores.
What's interesting to me is that I've been in fights, threatened with a knife, was in some hairy situations in Saudi Arabia, and other situations that were very stressful and didn't have that feeling. So, I don't think it's a given that it will happen under extreme stress. But I will say that it happens, and when it happens to you, you will know it.
I've seen it happen to others, too. Road rage is a fairly common one.