Many pet hates in this thread which totally correlate with what we experience daily, however, the worst is general attitude, something I find to be very prevalent with business types who feel that they are above everyone else on the train, including staff.
There was a guy on an XC HST last year who got on at Birmingham, had his ticket checked and promptly fell asleep. He then woke up as we passed Northallerton to realise that he wanted to be off at York for a change to Scarborough. Cue several loud swear words before charging off down the train to find the "manager".
A couple of minutes later I could hear his voice in the next carriage and he duly appeared, storming back to his seat with the guard following behind. The guard was a nice guy actually who explained that we would be arriving at Darlington shortly and there would be an East Coast service heading back to York in around 10 minutes with a FTPE connection at xxxx, all in all, he would only arrive home around 80 minutes later than planned. The guard would even make sure he was able to use the EC service free of charge as mistakes happen, people fall asleep etc etc.
However, this simply wasn't enough for the "business type" who ranted about owning a business, was enduring a busy month due to overseeing a new office opening and was basically above everyone else. Basically, as the guard had checked his ticket on leaving Birmingham, the guard should then know that this guy wants to be off at York and should make sure the train didn't depart unless he was sure this guy was not asleep on his train. So out of a good 300 pax who boarded at Birmingham, this man expects the guard to specifically remember this one person and his destination/change.
Cue a rant about customer service, a name request and the threat of sending a letter to higher management (who he claimed to know someone who knows someone) and then we were at Darlington where he departed and that was that.
I hope he did send a letter to XC and got a reply banning him from all future travel on their services, it's the least he deserved other than being thrown out of the window at 125mph!