So, let me start with an example: a flapjack in the UK is a baked item, made from oats bound together with butter and syrup/sugar/honey, often containing nuts and/or dried fruit. In the US, it’s a pancake.
Biscuits are a famous example too: a biscuit to a Brit is wide ranging umbrella term for cookies and cracker’s, while the scone-like American biscuit is a very different proposition.
The other half of biscuits and gravy - the gravy - is another. The various sauces known as gravy in the US would not pass muster in the UK. Nothing wrong with them as foodstuffs, but to a Brit, the word “gravy” means brown gravy.
Cider in the US is essentially a variety of apple juice. In the UK it is, exclusively, fermented. There’s no such term as “hard” cider to differentiate things in British English, because there doesn’t need to be, in the same way as there’s no point saying hard beer or hard wine.
In Newfoundland, goulash is elbow macaroni in a meaty tomato sauce, not a paprika-rich Hungarian stew!
Marinara sauce is not a well known term in the UK, but if it is used, don’t be surprised to find it full of seafood: the name can be taken as a reference to things “marine” in origin.
On a related note, my aunt was once famously taken pity upon by a kindly shop cashier in Austria in the 60s, who - quite correctly - surmised that a couple of foreign 20-something tourists had no need for two jars of Honey-brand floor polish, and duly explained that it wasn’t actually honey.
So, what else do we know about this folks? What have you accidentally ordered because the name meant something entirely different to you?