I think what made it tough is that it doesn't look like any open cluster I've seen before (except maybe, kind of similar to that open cluster contained within M24, the star cloud).
It looks like a smudge of light. In the 12" with a lot of power, stars could be resolved. Resolving those stars is what made me thing that we'd pegged it (that and the stars in the area that we hopped from).
It is appreciable, but it was also very much like the feeling you get after figuring out the trick question on a test.