Really annoys me when people sit their kids in the seats reserved for the elderly and disabled then fail to get them to move when someone gets on who obviously needs to be seated. Annoys me too when the driver gets going before someone who's tottery or on crutches gets seated.
The worst thing I've seen was when on a train to NCG, one of a group of lads who'd been intimidating me noticed he had s**t on his trainers and gave them a good wipe on the upholstery.