Amongst the older generation an historic, an hotel and an horrific are completely fine. We don't want to be sticklers for rules... although we do love telling people that they're wrong.
Quite simply the rule for a and an is based on pronunciation. You probably know that. It's not based on whether the following word begins with a vowel or consonant, but whether it's pronounced like a vowel or a consonant.
You can't say "an useful..." because even though it starts with the letter "u" the first sound is that of the letter "y". It has to be said "a useful...", so what about historic, hotel and horrific?
In a discussion that took place the other day, the people (well, women) in favour of an were educated in an all-girls private Catholic secondary school. Not that we want to open the debate of Catholic education, private education or even just single-gender education...
|"Hey look! An cathedral!"|
They were taught correctly at the time... unfortunately language does evolve and change and now they're wrong. We used to believe the Sun orbited the Earth... that's not the case now.
You can say "an hotel" if you want. As long as you come from a place where "h" takes a back-seat in terms of pronunciation and never shows up in words.
"In 'artford, 'ereford, and 'ampshire, 'urricanes 'ardly hever 'appen" is an 'istoric moment in the film My Fair Lady. However, putting an before historic, hotel and horrific, is a horrifically historical way to speak English, so get out of the habit!
We don't care what you were taught. This is the way things are now.