A Duplicitous Administration
If they lie, you can’t trust them. That’s a fairly straightforward rule. It is certainly the one that trial lawyers bank on.
It is not a hard and fast rule. A person may shade the truth for various reasons: vanity, personal allegiances, financial incentives, etc. Usually, once you figure out the relevant motivation, you can sort out on what matters he is probably credible and what he is prone to lie about.
Sometimes, though, the story is so unbelievable, so insulting to the intelligence, that a rational juror knows it is best to discount all of the testimony — or, worse, to conclude that the truth is likely the opposite of the witness’s desperate version.