The best part is when Americans find out that what they spell and speak isn't English, but American English. Proper English can be markedly different. And Queens English/RP different again. I blame Noah Webster
As an American Ex Pat, I find myself using a mix. Sometimes I type 'realise', sometimes 'realize'. In English, the "S" sound can be like a "z" - which is a "zed" here...er... it goes on and on...
Sometimes my "English" gets corrected, either by locals, the English, or Americans. If I am ornery(a word rarely used outside the US, I am finding) I usually ask them "In which English are you referring?" If they can't tell me, I ask them how it is they feel qualified to police the language.