The biggest faux pas stories I can remember all deal with my dad's brother, Paul. Paul was always a little weird, some of the weirder things that happened during holidays were when he:
1) tried to steal a silver baby cup from my parents' locked china cabinet. It didn't even belong to one of his relatives (it was from my mom's side), he just thought he could get some cash from it. My brother caught him destroying the lock with a small screw driver.
2) the year that he brought his new girlfriend over and proceeded to loudly make out with her at every opportunity. It felt like it lasted an eternity, but apparently it only took 45 minutes for them to firmly be told to knock it off or get the ehell out.
3) the year his next girlfriend showed up wearing just cutoffs and a bikini top. To Easter.
4) the year my parents got a call on Thanksgiving from a customer looking to speak to the owner of my father's business. Long story short, my dad's brother had been telling people he owned my dad's business, my mother was his wife, he owned their house, and I was his daughter. What a great Thanksgiving.
5) that time (the last time he was ever permitted to even have contact with anyone) when he cornered me in the living room and told me what a beautiful young lady I was growing up to be. I was 11. I bellowed, "DAD!!!", and my dad, his other brother, and my married-in uncle came running. I ragefully told them what happened, and Paul was summarily (read:forcefully) ejected from the premises. He had been a bit weird with me before, and that was the last straw. No one but his own mother has bothered with Paul for years, and even she dropped him last I heard. Bye Felicia, er, Paul!
ETA: As to #5: it was the *way* he said I was growing up to be a beautiful young woman; like, that completely lascivious, gross tone that makes you want to pull off your own skin. UGH. So glad I never have to see him again.