I had a customer provoke a "fight or flight" response I haven't had in years. He didn't do it purposely as it was just a "Thank you very much." pat on the arm. But because it was unseen and unexpected, I nearly jammed my elbow into the man's face in panic.

To the man's credit, he realized what happened and apologized profusely. I accepted (and apologized myself), as it wasn't intentional, but my nerves were shot for the rest of the day. They're still a bit shot today. It's been 5+ years since I've had that reaction and I'm nervous about my job. My boss has been good to me, but if she chooses to believe I nearly attacked someone for no good reason...