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.
Best Answer
There is a slight disconnect between what western languages and what the Japanese associate sincerity with.
This article about decoding asian business codewords explains the difference.
Increasing your ranking in the Sincerity skill allows you to maintain that face. An observer who sees through the sincerity would see the indecision and lack of inner harmony behind your statements. Failure at sincerity leads people to believe that your thoughts are incorrect - because they are disharmonious and have caused you loss of face.
A modern example would be a woman asking her boyfriend "Does my bum look big in this?". A man who says "You look wonderful." is being Makoto. His skill at Sincerity will determine how well that is received by his girlfriend. A man who says the opposite of what he really thinks is using deception.
In game terms, the first example is Sincerity (Honesty) opposed by Etiquette (Conversation). If the man fails loses to the girlfriend, the girlfriend thinks that he thinks the opposite and was trying to lie. If he beats her Etiquette roll, she accepts that that is what he thinks in the way it was intended.
Another example comes from internet-based argument. If side A has a factual and logical argument but makes the argument poorly or fails to connect with the emotional bias of side B, then he has rolled poorly on his Sincerity (Honesty). Side B continues to believe as he has been or believes that something is not the truth.