Got a wee bit of an English lesson in here. I always thought a before a word beginning with a consonant while an before a vowel was a cut and dry rule. Guess it's not that simple.
Blackxrose touched on cheque/check and colour/color. In Canada there are a few words that are spelled differently from American English, but are still correct spellings. Examples:
Colour/Color = Colour is how it's spelled in Canada. Color is American English.
Cheque/Check = It gets a little weird in Canada because cheque is the common spelling for bank terminology, like "I wrote a cheque" or "I'm depositing money into my chequing account". However, check is the proper spelling for things like "checkmate" in chess, "bodycheck" in hockey, "spellcheck" when on forums.
Center/Centre = Once again, this another weird one. Center should never be used at any time; it should always be spelled centre, as in "the centre scored a goal" "the centre fielder caught the ball" "drill a hole in the centre of the board" "I went to a conference at the MyFreeCams Centre"
Tire/Tyre = This is the word used when referring to the rubber object that's on an automobile. In Canadian and American English, "tire" is the proper spelling, while in the United Kingdom "tyre" is used. What I don't know is if the British spell the word "tired" as in "I'm feeling tired" with an i or a y.