I used to work in a library and when shelving the fiction books I would put them in order on the trolley first. Without failure, every single time I went to shelve a few at least one person would pounce on the trolley, pick up a book and would either dump it back on the top, or put it back in the wrong order. Sounds like a little thing, but it was so annoying!
Not really annoying, but one day a woman wanted books to help explain 'the birds and the bees' to her son and after I showed her where to find them she spent ages going on about how he had been asking a lot of questions and she wasn't sure what to say to him, how he was at 'that age'. It was a really random conversation to have in the middle of the childrens section!
Also, I worked at Vision Express for a few Saturdays and on my first day they literally put me behind the counter and told me to deal with people, without giving me any sort of training. The second week I was there a woman came in wanting to try some sunglasses on, but they were locked onto the stand and I didn't know where the key was. The other staff were all busy, so I explained that it was only my second day, I hadn't been told where the key was, as soon as someone was free I would get them to help her. I went back behind the counter, where a couple of minutes later I saw the woman giving me a dirty look, so I went over and repeated what I had said before. Another member helped her a few minutes later, and everything was fine. Until the week after when my boss said the woman had put in a complaint about me, saying I just stared at her and didn't go over to ask if she needed any help, which was blatantly not true. However, my boss didn't like me and it was pretty clear she didn't believe me when I explained what had actually happened.
In the job I do now, I don't have to deal with customers, but some of the staff are horrible, especially considering that they work in care and have done for years.