[IshKebab]

It is one of the few things in language that definitely is wrong.

The point of saying "an" instead of "a" is to make it easier to say a following word that starts with a soft sound (there's probably a technical term).

"A igloo" is hard to say, so we change it to "An igloo" which is a lot easier.

"A history lesson" is not hard to say so there's no need for "an".

Have you ever said "an history lesson" or even heard anyone say that?

No. Of course not.

[JoshuaDavid]

It depends on pronunciation, not on the first letter. This is why you say "an MBA" not "a MBA". If you are from a region that pronounces "historic" as "'istoric", then "an historic occasion" sounds fine and "a historic occasion" would sound weird.

[sfn42]

I would assume it has to do with the next word. You wouldn't say "a occasion", you say "an occasion" so then it's natural to say "an historical occasion" as well.

Not arguing correctness, just intuition.

[eynsham]

> Have you ever

Yes, often.