That's not a clear-cut mistake, and in my opinion totally forgivable. http://www.betterwritingskills.com/tip-w005.html
I didn't knew why I always used 'an hour' instead of 'a hour' but that's what I was taught. The link explains a lot.
People keep correcting me for typing 'an hour' instead of 'a hour'.
http://grammarpartyblog.com/2011/01/10/an-historical-event-i...