I find it constantly annoying when staff say they 'can't' do something when in reality the answer is they don't know how or are simply unwilling. This is usually followed up with some sort of patronising quip about how many decades they have on the job.
My last was Bristol Temple Meads ticket office who refused to sell me an over distance excess. They claimed it couldn't be done, that I'd need to buy a single ticket from my destination to my new destination, and when I politely explained it could be done and there was no need for a single ticket as well I got the whole 'I've been doing this 25 years' thing.
The staff on the gateline, equipped with Avantix mobile machines, were happy to issue one subsequently. Madness.