Tell me your biggest mistake.

These days, fewer people ask this question.

Be ready with a response, though, that demonstrates that you really did encounter a challenging learning situation—some form of challenge to go past—and that you truly did learn something from that experience and have incorporated it into how you live today. This demonstrates resilience, which is a valuable quality in consulting. To put it gently, not all consulting projects go off without a hitch or run without a hitch. Your career and interview process may benefit from your ability to "turn lemons into lemonade.