My pet peeves are people who sit in my assigned seat and assume I'll be OK giving it to them, as on a recent ORD-ATL operated by a E175 I booked 8A (first row of Y) which has good legroom to strectch right under the F seat infront as there's no bulkhead wall compared to 8C which has the issue of the drink cart slamming into my knees. Asking courteously the woman seated in my seat she reluctantly had her husband take 8D and her in 8C and during the boarding process she tried every trick in the book including flirting to try and steal my 8A. Guy in 8F was tired of her .....ing so he took 8C and had the couple take 8DF. I assume the couple wasn't supposed to be seated in Main Cabin Extra to begin with and decided to self upgrade which AA seems to allow as the perk is free booze, and inevitably they decided it would be a great idea to get hammered on a 600 mile flight.
If they'd have sat in their original seats and asked nicely then I would've thought differently.
Another pet peeve is people who do not bother to shower before heading to the airport... Even worse than the above