What is the tu quoque ("you too") fallacy, and why is it a flawed form of ad hominem?

Dismissing someone's claim by pointing out they're a hypocrite — their behaviour contradicts their advice — as if that made the advice false.

"Tu quoque" is Latin for "you too." The pattern: A makes claim X; B replies that A's own past statements or actions are inconsistent with X; therefore X is false. It's a sub-type of ad hominem. Hypocrisy is a fact about the person, not the claim — a chain-smoking doctor who tells you "smoking is bad for you" is giving you correct information regardless of their own habit.

"You're lecturing me about saving money, but you just bought a new car — so your budgeting advice is nonsense."

The advice stands or falls on its own merits; the speaker's inconsistency may make them a hypocrite, but it doesn't refute what they said.