A liar will not be believed, even when he speaks the truth.

Credibility is a single account and every withdrawal comes out of the same balance. Once somebody has been caught shading things, listeners stop assessing each statement on its merits and start assessing the speaker, which means even accurate reports get discounted. That is expensive in a way that is hard to see until you need to be believed.

There are two practical implications. Guard the small honesty, since a reputation is built from minor moments rather than dramatic ones. And if you have damaged yours, the repair is slow and mostly consists of being verifiably right about unimportant things for a long time.