When I was in high school, this girl came in to class with ash on her forehead, and I had no idea what it was.
So I said: "Uh, [her name], you have something right there, on your forehead"
She looked at me, and kind of smirked, so I said "...yeah"
And I think she realized that I had no idea why she had ash on her forehead, and she was like "Uh..it's ash wednesday"
And I still cringe thinking of that exchange.