Once interviewing a developer I asked her what was the most challenging work from her experience. She said it was writing a game of tic tac toe, I paused for a bit, and asked her if she can describe in more details which part of it was challenging.

She said, "I am not too familiar with the rules of the game so it was hard".

The whole room went brain-freeze for 10 seconds before anyone could find a way to continue the conversation.