Here's what they said:
1. Calling someone "thick":
"Describing someone as 'thick' means something very different here in the UK too. It’s a slang word for a person of low intelligence!"
2. Saying hello to Londoners:
"SAYING 👏 HELLO 👏 IS 👏 ONLY 👏 UNACCEPTABLE 👏 IN 👏 LONDON 👏 BECAUSE 👏 EVERYONE 👏 IS 👏 MISERABLE."
3. Asking for a "rubber":
"Asking for a rubber means an eraser in British (and Indian) English, whereas in the US it sounds a bit like asking for a condom."
4. Complaining about the food:
"Complaining about the food you were served. Being a Brit, I just accept what I got even if it wasn't quite what I asked for."
5. Saying "her" or "she":
"Americans will say 'she' or 'her' about someone who is right there, which in the UK is kinda rude. You'd use their name, or if it's a customer, you would say 'this customer' or 'this lady,' not 'she.'"
6. Saying "crap":
"My mum is originally from the UK, and I was born in the US. Growing up, I said the word 'crap' and when I first said it, she was appalled. In England, saying 'crap' is like saying the word 'shit.'"
7. Saying "double fisting":
"When my American friends said I was 'double fisting' because I had another pint before my other was finished, I thought he meant something else entirely."
— u/Vehlin
8. Having someone hold the door of the tube:
"My British partner and I (American) were in London, running late to meet our friends. Just as we get to the tube station, I see our train has just pulled in. We haul ass across platform and I yell, 'HOLD THE DOOR!' Someone does, we make the train, I don’t see a problem. My partner, by contrast, is mortified. This was 4–5 years ago, he is still mortified. Apparently, we were meant to just let the train leave without us and wait for the next one?"
9. Cheering during exciting parts of a movie at the theater:
"Going fucking wild at the cinema during a hugely popular film like Avengers: Endgame or a Star Wars film. There’s a bunch of videos on YouTube of the audience reactions to various big franchise films and I don’t know how anybody lasts more than a few minutes in that room."
11. Loud talking:
"Talking so loud that everyone within 100 meters can hear what you are saying. Whooping. Just don't, Chad, you aren't on a buckaroo now."
12. Chanting "U-S-A":
"It's socially acceptable but horrifying when Americans chant U-S-A at any given opportunity."
13. Calling someone "sir":
"Addressing a stranger as 'sir.' The only person you address with 'sir' in the UK is your teacher."
14. Asking "How ya doing?"
"Americans will ask 'How ya doing?' but not really expect an answer."
15. Freely voicing disapproval:
"Voicing your disapproval of something. Obviously, it's not US-exclusive, but you guys are way more likely to call out on it, whereas if a guy was drinking too close to me, I certainly wouldn't say anything to the bloke. It's his business."
WOW!!!!! Well, Americans, tell me what UK people do that horrifies you when they come to the US! Let me know in the comments!
Responses have been edited for length and clairity