As an Aussie writing books that are (for the most part) set in the US, I have to be very mindful of using US colloquialisms and not interject Aussie-isms. Even stories that I base in Australia need to be understandable for those overseas. I am lucky that I have a terrific beta-reader who is based in the US and she helps immensely. I still like to ensure that I give her as little to fix as I can though.
There have been a number of times though when I have merrily gone on my way completely unaware of the differences in speech between the US and Australia: footpath versus sidewalk, boot versus trunk, crib versus cot, notes versus bills, bonnet versus hood, grey versus gray, just to name a few. There are still some words that I struggle that catch because they are so ingrained in my vocab that I don't even notice I'm using them until I get a comment or correction in my manuscript. And I can still remember her reaction to me saying that someone was telling porkies and the first time she encountered fairy floss in one of my stories rather than cotton candy.
In my day job, my boss is British and we've had conversations about Brit versus Aussie sayings. Luckily as a lover of British TV, I can understand a number of his expressions. Although the first time he busted out an "Are you a Billy?" when I was alone in my office, I was lost. He meant it in reference to the saying "Billy No Mates" which I'd never heard before (in Australia a similar saying would be "Nigel No Friends" - I'm sure you can guess what it means).
Although I'm no etymologist, the subtle difference in language from country to country, and even region to region fascinates me. I can still remember being a young girl moving schools from South Australia to Queensland and being told to put my port on the port racks and I had no clue what they were talking about. Then of course, there is the language of fandoms and the internet which is a whole other thing again.
I'm curious, what expressions (either from other countries or your native one) have you encountered that left you dumbfounded or that you had to explain to others?