Okay, this one isn't a comment in code, yet it amuses me anyway. This is Exercise 24.1 in The TeXbook.

Can you think of a reason why you might want ‘A12’ to be a ‹hex digit› even though the letter A has category 11? (Don't worry if your answer is “no.”)

For the record, my answer was “yes.” Unfortunately, my reason wasn't correct.

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