Just for funsies:
What's etiquette breach have you committed in your life? Past sins that you now regret are A-okay!
What's the worst etiquette breach you've wittnessed in a social setting? (as opposed to customer service, etc etc)
I asked a bride and groom "Do I really need to send in my reply card? You know I'll be there". I didn't even realize it was rude until now and want to call her 4 years after the fact and apologize!!!
One that I see constantly is when people go out and share bottles of wine they split the check evenly and make those who didn't drink pay for their alcohol.