As a Canadian, one of my difficulties is proofreading, because I have been exposed to English spelling in school and American spelling through books. So I find myself using both and then having to correct many words during my editing process. Spellcheck does not help, because its base dictionary is American. Further, it is oriented to business rather than literature; therefore, its spelling and grammar functions (and autocorrect) are unable to differentiate between accidental errors and creative licence.
On top of that, I do misspell words at times, or my fingers type something other than what I intend.
The bottom line: Proofreading requires correcting not only general mistakes but inconsistencies in the type of English I use.
Perhaps it’s stubborn of me, but I do try to stick to Canadian/British English. Especially for stories set in Britain or Canada. Will that confuse or annoy American readers? I do not know.
Do you notice spelling errors in books and articles? Do you ignore variations in the language as written in other countries?