Snowy said:
But that leads to no chance of a misunderstanding ever (bar wisdom penalties and no ranks), you can't always convince some people of the truth no matter how hard you try.
People can still roll 1s...seems like someone rolling a 1 on Sense Motive is the cause of most misunderstandings. Circumstance penalties for hostile or suspicious listeners could easily render a Sense Motive total in the negatives too.
I'll elaborate.
If you are being honest, but you need to convince your audience of the truth, then you take zero on a Bluff roll (the lowest you can take for this purpose, representing being as sincere as possible, looking them in the eye, etc...) or take 10 on a Diplomacy check. The audience generates a Sense Motive total, with situational modifiers applied...
Audience is under effects of Antipathy or similar magic: -20
Audience is Hostile: -10
Audience is Suspicious:-5
Audience is Indifferent: 0
Audience is Favorable: +5
Audience is Friendly: +10
Audience is under effects of Charm, Sympathy, or similar magic: +20
Speaker has lied to audience in the past: -2 per lie (maximum -10)
Speaker has reputation for honesty: +1 to +5, DMs discretion
Speaker is Chaotic and has reputation for it: -3
Speaker is Lawful and has reputation for it: +3
Speaker is unknown to audience: (Audience's best possible attitude is automatically Suspicious, unless they are idealist simpletons.)
Speaker is known to be a member of a Lawful police, religious, or military organization, or is wearing the uniform of same and has made a successful Disguise check vs. Audience: +3
Speaker is known to be a member of a Chaotic police, religious, or military organization, or is wearing the uniform of same and has made a successful Disguise check vs. Audience: -3
(I'm sure you can think of others that would apply.)
Since the desired outcome is the audience believing you, failure of their Sense Motive check results in disbelief. You still keep your Bluff rating because no matter how sincere you're trying to be (taking zero), your tongue is still as silvered as you've trained it to be. It might seem strange to insist on taking a zero to be honest, but Bluff is all about being dishonest, so there should be a penalty. Diplomacy is far more effective, hence, 10 may be taken.
The numbers for magical effects and for Friendly/Hostile might seem high, but remember that strong emotions can play a large part in affecting someone's willingness to take you at face value. Someone under a Charm effect might believe their charmer if they told them the moon was made of green cheese, while someone that hates your guts (Hostile) might not believe they were on fire if you told them even if they could smell their own charred flesh and it was making them jones for bar-b-q.
(Sorry to get all pedantic there.)