Have you ever done or said something while having sex that completely killed the mood?

I once dated a girl who worked at, and was the daughter of the owners of, a restaraunt that I frequented. Her parents did not approve of our relationship as I was not Greek (thats another story though)... so for the most part we had little interaction with each other while in the restaraunt.
So one day while Im there I notice one of the other regulars talking to her and he gives her a little swat on the butt before he leaves. This bothered me slightly, but wasnt anything I couldnt get over.
Later that night as we are going at it I give her a good spank and say something to the effect of "So other guys can put their hands on you but Im not allowed to so much as look at you?" In all reality I wasnt upset about the event but apparently the stress of keeping our relationship somewhat secret had caught up with her because she suddenly started crying! It was very awkward and I felt terrible about it even though I wasnt serious about being mad. Needless to say, nobody got off that night.

How about you?