Have you ever had someone lodge a letter of complaint against you?

8 weeks into my new job and I got my first. (In this job at least) The complaint was unprofessionalism by dropping the F-bomb and not getting back to the person with the information she wanted to know. (I got the feeling it was really the last half of the complaint that she was worried about.)
Well the manager took me into the office to address the complaint to me and asked me if I remembered it. I said I didn't, but it sure sounds like something I'd F'n do... It did not go down well.