It might not even be an error. It depends. I think I know the difference, but like Jmat I need to check to be sure. First I'm going to say what I think the difference is, than I'll check.
I think that "an historical event" simply means something factual, something that really occurred, something that is in the historical record, while "an historic event" means an important event that "made history."
Yes, I'm correct.
Well, every "historic" event actually occurred, so all "historic" events are also "historical."
The importance of any event is a matter of opinion. Any "historical" event can be "historic" to someone if they think it is important. It is historical fact that Idaho achieved statehood on July 3, 1890. Is it historic as well? Yes, if you live in Idaho, probably not if you don't.
The only way I can see the use of the "wrong" word as being an error would be when talking about the present day. It would be correct to say "The decision handed down on June 26th by the Supreme Court of the United States was historic," because it was so important.
In my opinion, it would be incorrect to say "The decision handed down on June 26th by the Supreme Court of the United States was historical," because it is still news rather than history. On the 1-10 scale I'd call that a 2. Maybe 1.5.
ahdictionary.com's usage note says "These distinctions are not always observed, however, and a historic tour of a city might include the same sights as a historical tour."