MRP spied this recently:
And felt compelled to ask, “Does anyone actually believe that an historic is correct?”
Thanks to Paul Brians, who pronounces:
You should use “an” before a word beginning with an “H” only if the “H” is not pronounced: “an honest effort”; it’s properly “a historic event” though many sophisticated speakers somehow prefer the sound of “an historic,” so that version is not likely to get you into any real trouble.
But Bill Walsh has a full exposition here.