One thing which infuriates me is how hard it can be to find out a single fare - and so it's impossible to tender the correct fare, unless by chance you have it. I rarely ride on buses, but I took a journey with my nephew last Friday, on Arriva. In my mind, drivers don't like notes being tendered (maybe that's a hangover from the old days) so I checked the company website - no information on single fares. So I tweeted them - no response. Finally I called - no reply. As it happens, the fare was so expensive that a note was fine, through sheer luck.
Lack of information is a real achilles heel for companies which directly rely on being given cash by the general public, and I think it puts occasional riders off.
Separately, I was once refused at a Scottish supermarket when I tried to pay with a Northern Irish note. I think I used the phrase 'pot, kettle, black'.