For me it has to be... drying clothes in the USA
See, there is this one teeny-tiny sentence in my comic, in which a character says that he picked a pair of boxer shorts from the "clothing line outside". When I wrote it, I didn't really think much of it. You wash your clothes, you put them on a clothingline and let them dry, end of the story. That's how it works in Italy. There's absolutely no reason it'd be different in the USA, right?
...Wrong :'D
Some months later, I stumbled upon this podcast in which an Italian expat talks about US culture shocks and I found out that... actually, most people in the US have a dryer for their clothes. And some places downright put a BAN on clothing lines. I did some research and found out that there is quite a lot of debate over the "clothing line vs dryer" issue. I also found out that, in cold climates, clothes can downright FREEZE outside. Living in an area that is fairly hot and where it never snows, I admit that I never, ever thought of that being a possibility :'D
I eventually decided to leave the clothingline sentence where it was in my comic (my character lives in a fairly isolated area which probably wouldn't be subject to strict HOA rules + he doesn't have a whole lot of money to spend on fancy appliances), buuuuut... the whole experience definitely teached me to look more into potential US culture shocks I may have not considered before