Aside from the obvious ones like hour, herb, etc., this is the only one I can think of. E.g., I'd never say "This book is an history of Egypt." But "historic" gets the "an" and the "h" becomes silent.
Maybe it has something to do with the accent being on the second syllable of "historic" but not of "history"?
Nyarpy, no! Not herb!! That's purely American, sounds mad in English English. As I've said elsewhere, I think not pronouncing the H in historic/al feels pretentious to me, as if trying to sound French. But, I know, we don't pronounce the H in "hour"... There isn't much logic.
Both "a historic" and "an historic" are fine. "An historic" is a little more dated now, but is still heard, at least where I live.
It does seem a little odd to have Duo tell me one of them's a typo though, particularly as it seems to choose a different one in different exercises.