Me: That's a pretty Christmas decoration.
She: That's not a Christmas decoration! It's for Honecker.
Me: For who?
She: For Honecker.
Me: Huh? You mean Erich Honecker, the recently-deposed East German leader?? Someone's putting candles in the window for him? But why would--
She: (sounding more Lower East Sidey than normal): I said Hanukkah, you fool!
Me (humbly): Oh. I see. [Pause] What's that then?
Of course, by my 27th year I ought to have known about Hanukkah. I'd passed my PhD viva just a few weeks earlier, and how can someone with a PhD not know about a major religious festival? But I didn't. All these years, that little nugget of ignorance had been waiting - waiting to be exposed by wind, rain, and the right combination of chance circumstances and make me look foolish.
But I'm not alone. A few years earlier I'd spent an hour trying to convince an otherwise well-informed woman that the tides were affected by the movement of the moon. It was a fact she'd just never happened to come across, and I must admit, the more I assured her that it was true the more shrill and unconvincing it sounded even to me. I don't think I persuaded her.
We all have them, I expect, these depth-charges of ignorance, ready to explode in our faces just when we're in the presence of someone we particularly want to impress.
Don't we? Are you seriously telling me that this kind of thing has never happened to you?