Don't you hate it when someone spends an hour attuning to a magic item before anyone can stop them?

A dihedron entity float down from the empty hole of a ceiling above your party's heads. On all sides it glows the same mysterious rune.

The barbarian rolls to announce he recognizes the rune. An int check is required. He succeeds the roll - "I recognize those runes! Each is a twenty."

DM: "You didn't need to roll for that observation" Player: "Yes, but my character would have thought it was a moment of intelligence."

My mom who's barely keyed into stuff like this laughed her ass off, thank you.