I agree with @Amparo about still needing to learn the lingo, even when both countries speak English. I’m a British Canadian, and still stumble over the different words at times.
I’ll add to @Amparo’s list for UK/US English:
- The doctor’s surgery is the doctor’s office - not a procedure involving a scalpel.
- First floor in the UK is the ground floor in the US. Important if stairs are an issue for the sitter.
- A joint means a roast of meat - nothing to do with drugs.
- Pavement is the sidewalk, not the area where cars drive.
I know there are lots more, but those are the ones that first come to mind.