When you ask someone for a dance and they decline - only to get up and dance with someone else. That one gets my vote.

"Dancing is a contact sport; football is a hitting sport" - Vince Lombardi

Granted, I agree with everyone who believes that refusing one partner and then dancing with another is downright rude.

However, what about dance floor etiquette? A story you wouldn't believe:

I was dancing a cha-cha with my partner at a social. Another couple was very near us on the floor, practicing their cha-cha routine. This other couple kept on bumping into us, quite aggressively, I must admit. After the 4th or 5th time, my partner got so angry, he reached around me to kick the other gentleman and kicked me in the process. I immediately told him how wrong he was. He apologized to the other gentleman. It took him about an hour to apologize to me.

Talk about rude...