I grew up very poor, but my mom's childhood friend was well off and had rich friends. I would stay with him a lot so his kid and I were like family to each other. A rich "daddy's girl" friend of his, who I had only met once or twice before, apparently had a crush on me and sent me a formal invite to her quinceañera (sorry if misspelled) with all expenses paid. On the weekend of the party they had a black car pick me up from school (Friday after last bell). It drove me to a shop where I was fitted for a suit, then a stylist for a haircut and my first manicure, then taken back home. The next morning (Saturday) I was picked up from home and taken to the local airport where a private jet was waiting for me and the new suit was already in its coat closet. I was flown to Miami and put up in what seemed like a really nice hotel. The party was glitz. Everyone, including myself, was wearing something that cost thousands of dollars. Definitely no rented suits there. The next day, I was flown and driven back home. After going through all of that, I spent literally five seconds with the birthday girl during the party before she was whisked away and I didn't see her again.
¯\_(ツ)_/¯

It was a surreal experience.

EDIT: As an adult I have had many situations that qualify for this post, but I never felt as out of place as I did that one time.