What happens when one PC tells the truth to an NPC about something and another PC bluffs to that same NPC and the NPC has failed their sense motive?
Specifically I reported to an NPC guard that one of the PC's had stolen the ring from a dead person which was to be returned to the family. This other PC then lied claiming it was a family trinket. The NPC failed the sense motive against the bluff. The NPC never did sense motive on me (the person telling the truth) that I am aware of.
The description of the Bluff skill reads:
Deceive or Lie
If you use Bluff to fool someone, with a successful check you convince your opponent that what you are saying is true.
The statement the Bluffer was making was "The ring does not belong to him, it belongs to me". The NPC now believes that lie, so in corollary the NPC is now also convinced that the previous statement was false. The fact that the NPC already had information about the ring which contradicts this lie should have been represented in form of a negative modifier to the Bluff check.
The DM might decide that this situation is a bit unfair. So she might want to treat this situation as a fair contest between your two characters. This could be done by ruling that the guard is not really sure whom to believe. She might have the guard drag both characters to court where they can then convince the judge about who is the rightful owner of the ring. Rule-wise, this could be represented with opposed Diplomacy checks between the two characters.
Abe the PC told the truth to Carlos the NPC guard; no rolls need be made to tell the truth. Bill the PC made a Bluff skill check to lie to Carlos the guard, opposed by Carlos's Sense Motive skill check. Bill wins, and Carlos believes Bill's lie. But Abe's truth and Bill's truth are at odds, and Carlos believes them both! Carlos knows that either this is all a hilarious duplicate-ring (or time-travel!) mixup or one of them is lying. This GM supposes that, as a city guard, Carlos will opt to think the latter.
Abe, though, has offered Carlos the guard a truth that can be verified. Carlos should take the ring, investigate its provenance, and either return the ring to the owner if Abe's story proves true (which it will) or return the ring to Bill if proves false (which it won't unless Bill's laid the groundwork for that beforehand). If the ring goes back to the owner, Bill should keep an eye out for Carlos the guard—he'll likely be suspicious of Bill when next they meet.
If for some reason Carlos the guard can't take the ring, his best bet is to make Abe and Bill repeat their stories. Several times. This'll force Bill each time to make another Bluff skill check to lie that's opposed by Carlos's Sense Motive skill check. Eventually—just like in any interrogation scene—, Bill will screw up (rolling low), Carlos will detect the fib (rolling high), and the jig'll be up for Bill (unless, y'know, Bill's lawyer arrives first). Abe'll always be in the clear, though, so long as he consistently tells the truth.
However, it's possible that because of the wide span two characters can have in opposed skills that there's no mathematical way for Carlos to penetrate Bill's ruse. So even after Carlos listens to their tales a couple of times, he can't resolve believing both Abe and Bill: Carlos is out of his depth. Carlos needs to haul these perps to the station, kick this incident upstairs, and have better-skilled interrogators go to work on Abe and Bill. (This GM tends to populate the city guard with at least one high-level commoner with feats and so on sufficient to make the dude ultraspecialized in the Sense Motive skill for exactly this sort of problem.)
The Bluff should have been influenced by the information you had given the guard and this would govern what the guard ends up believing, but how it plays out afterward is up the the DM.
As per the rules for Bluffing, if a lie is unlikely or far-fetched it receives a negative modifier of -5 or -10, respectively. Given that you had told the guard the ring was stolen, I would use one of these depending on other details of the campaign. Your DM could have also given the guard a Circumstance Bonus on his Sense Motive, depending on other factors. Ultimately, though, once this role is complete the guard's mind is made, at least for now. Note that Sense Motive checks cannot be repeated until there is another Bluff.
