The 2 responses I usually get are (1) I didn't know you couldn't bring that with you and (2) I didn't know it was in my bag.

I've had one individual bring a survival knife with him. It was found on x-ray. When pulled from the bag, he stated that he was sure that it wouldn't be found so he brought it. I thanked him for allowing us the opportunity to search his bag and directed him over to the officer that was standing by him.