I don’t know where this came from or why it seems to be proliferating, but please take a look at this Google News search for “an historic.” As I write this, this search returns 3,144 results for this exact phrase.
I know this isn’t any of you, but if you know someone who’s doing this, beg him or her to stop!
The rule you learned whenever you learned it still applies. Use an in front of most words beginning with a vowel sound, such as honor, apple, and irritating. Use a in front of everything else. Here in the United States, we pronounce the /h/ sound in front of words like historic, humiliating, and heroic, so those words take a.
Now please do what you can to reverse the tide of this, an ‘orrible practice if e’er I ‘eard one!