The specific example I am working with is the Sincerity (Honesty) skill in L5R but there is a certain factor of curiosity for any other system:
How do you really roll to tell the truth and logically fail?
After all, if someone is trying to "sense motive" (to use D&D 3.x terminology), and their target is telling the truth, how does it mechanically work for them not to believe? No books really cover this because they tend to play to the side of discerning a lie, which is easy. The liar needs to win the contest to be believed, and everything works very intuitively.
If I want to convince someone I'm telling the truth, don't I want to flub my sincerity roll so their investigation roll is higher and they know I'm telling truth? That doesn't seem to work especially because better stats make your truth less believable.
Granted, I always want to roll high, but how would the investigation roll be affected? If I win I am obviously telling the truth, if I lose I am obviously telling the truth. So in a well-meaning ruleset, what can be done to go about a character being mechanically bad at telling the truth?
Side Note: For lies I sort of do a blind test. I ask the player what they think going in, let them roll, and tell them whether they feel the same way after the roll. It doesn't tell them if it passed or failed, they only have their roll to guess with.