The paladin follower of one of the major NPC's started acting different in the last two sessions. I didn't point it out due to no one beating her bluff check. Subtle changes in her aggression and attitude.
After two serious losses in the battle for the city, they were getting suspicious as to the enemy being too prepared. Again, no one beats the bluff and no one's really eyeing the paladin, who's taking a more and more active role in the planning.
The end of last session, while they're rallying the troops for a big attack, the paladin's orders were finalized. Kill the commander NPC. She moves up (Still bluffing), but is noticed by the ranger that her hands on her weapon. Everyone's looking around to what she saw (still can't beat the bluff with any sort of insight that it's her. everyone's trying perception). She draws her weapon and stabs the commander through the chest, then breaks for a run.
The party chases into an alleyway and finds nothing but the "skin" of the paladin. The doppleganger and some allies try to kill the heroes through some lurking attacks (in hopes of dropping one for her to assume the skin of). They almost failed at the skill challenge, but located her and began the combat. Thanks to the enormous perception of the ranger, even invisibility wasn't much use (set her on fire with his bow, which was pretty awesome). Once they captured her, they were able to interrogate the doppleganger on what the enemies plans were.
Heading into the next session, they're setting up an ambush, though they don't trust ANY of the paladins, wary of another spy...
It worked out awesome.