what was your worst incident you've had with a customer?

The worst incident I've ever had with a customer was one time I was helping a customer find an item in the backroom, before I could get to the backroom another customer asked me for another item and I've told the second customer I'm in the middle of helping the first customer, then when I've tried looking for the initial customer the second customer got so livid at me that he started cussing me out like a dog, I've decided to have a more experienced employee help the second customer while I continue with the first customer.

Oddly I've seen the second customer's wife (she was with him during that incident) a couple weeks later and I've apologized to her for what happened and she forgave me and admitted to me that her husband was an a-hole.