Arwen, I think the worst time to be proposed to would be when your sister was being walked down the isle at HER wedding. If insensitive bf hollered for everyone to stop as he had "something he needed to say" and then proposed, your sister would be mad that her special day was one upped, you'd be mad and embarrassed, and..... nah, ViolaKat is right, I'm sure. The funeral would be the worst if you were close enough to the person to actually attend his or her funeral.