English in the US versus the UK
In visiting the UK, one of the biggest surprises I've encountered is the differences between English spoken here and in the US. Sure, I knew there were some differences in vocabulary and grammar, but I never realized just how much they can affect communication.
Let's start with the obvious differences. Brits spell "color" with a "u," while the Americans leave it out. And don't even get me started on "favor" vs "favour"! It's like they just want to confuse us.