Well, I have a bit of a strange one here. I've never told anyone about this because it was just so weird and I don't know if I can convey it well enough. I'll try!
I was a student in a big city and I was just entering the main library in town to do some study. My arms were piled high with files and books and there were steps to navigate, and heavy swing doors, plus lots of other people. It was a busy place.
I remember opening one of the doors to go through it and, knowing there was a man behind me, I held the door open for him. He said, "thank you" and I went on my merry way.
A few moments later, I was on a second stairway, making my way to the music section, when said man appeared again and stopped me on the stairs. He said, "you didn't say 'you're welcome.'" I was confused at first and said something along the lines of "I'm sorry, I don't follow", and he repeated himself and stared at me. I realised then that I had neglected to say "you're welcome" when he had said "thank you" when I held the door for him.
Stunned at this bizarre turn of events, I apologised and said, "I'm sorry - I've got a lot on my mind" (referring to my studies) but he just stared at me again. Eventually we went our separate ways, but the encounter has stayed with me ever since.
On the one hand it was incredibly weird because people in big cities hardly ever hold the door for anyone in the first place, so it felt very weird for him to be admonishing me for anything, since I had held the door open for him. The fact that he had said "thank you" would, under normal circumstances, be enough for most people to move on!
Something else struck me, though. He was well dressed, well spoken and very blonde in a sea of people with brown or dark hair. Most people were dressed in casual clothing but his were quite formal. I should also point out that he was not old. He had quite a youthful face and could have been in his late 30s or early 40s at most.
His manner was not at all threatening - it was admonishing, like a father figure. I certainly felt as if I had been gently told off. It was very strange and almost as if he were from another era...