God, I’ve got so many:
My Brooklyn born-and-raised mother refers to a rude, cheeky child as “fresh”.
In the New York/New Jersey area where I grew up, plural “you” was almost always “you guys” or, if you were low-class, “youse guys”.
When I started working in my law firm in London the “you alright?” question threw me at first. It took me a while to stop replying with “yes, I’m fine, thanks, don’t I look ok?”
It was really when I started potty training my DS that I realised I needed to adapt my speech: when a two year old needs to wee quickly, commands need to be crisp and precise. At his nursery he was taught “trousers down, then pants!”. If I said “pants down” without saying “trousers” first it would have led to a whole puddle of confusion.
Similar to that, Americans often find the English/British habit of being direct about what exactly they need the toilet for (I’ve heard law-firm partners excuse themselves from meetings saying I’m so many words they need to wee) rather vulgar. But then you never hear in the U.K. anyone referring to a dog weeing on the carpet as “going to the bathroom”.
I almost got into a bit of trouble when writing my MIL’s eulogy and inserted the expression “she didn’t give a toss”. My SIL nixed it before the final draft - why DH couldn’t have warned me it wouldn’t be taken well I don’t know...