haha. I'll pile on as well. I wrote "an historic" and it said that I have a typo. No, it wasn't. I meant to write "an historic".
What is weird is that many people are having the opposite problem. Evidently they wrote "a historic" (and presumably meant to write "a hsitoric" so it wasn't a typo) and they are being told that they have a typo.
I've long known that duolingo has trouble distinguishing between a typo and an error, but this is the first time I've seen cases where if I write A it says B but if I write B it says A.
As for "a" or "an" here, I decided to look into it after I saw this thread and I learned that either is acceptable in English. Although for me, "an historic" does seem correct. According to Merriam Webster, "a historic" is a bit more common. Chicago Manual of Style agrees. Either is correct, but "a historic" is slightly more common since 1939. Before 1869, "an historic" was more common. In between they were about equally used.
According to a google Ngram viewer, the usage ratio is currently about 10.19 to 3.62 in favor of "a historic". It would seem that you and I are not very stylish, maria960299.