What does "That was his biggest mistake" suggest?

The statement "That was his biggest mistake" suggests that the action in question had significant negative consequences or was a major lapse in judgment. When describing something as "the biggest mistake," it implies that among all of the person's errors, this particular one stands out as the most serious or impactful. This is indicative of a substantial issue rather than a minor error, which aligns perfectly with the interpretation that he made a significant error.

Additionally, this phrase doesn’t support the idea of minor errors, a lack of mistakes entirely, or an intentional action, as it focuses on the severity and importance of one specific mistake rather than downplaying or denying errors altogether.