Anyhow, a nice example of this 'assuming it's standard' behavio(u)r came up on recently on the (AmE) copy-/(BrE)sub-editors' blog The Engine Room. There, blogger JD admitted to having believed until recently that Americans spell cemetery "sematary" because of the spelling in the title of the Stephen King book, Pet Sematary. In the book, one is supposed to understand that it's misspelt because children wrote the "cemetery's" sign.
That reminds me of being informed by BrE speakers that "thru is the American spelling of through". No, it's not. It's an abbreviated spelling form that is used mainly on signs (or painted on a road surface), and thus it's become the typical way of spelling it in drive-thru. You won't see thru replacing through in American newspaper articles (though it might be handy for an occasional headline--but I cannot recall seeing it in any) or novels--and you'd better not use it in essays for school/college/university.
Do you have any stories of misunderstood intentions due to "it must be the way they say it in American/British English" assumptions?