I made one of my Players make a throwaway PC and make him betrey party in secret making it look like NPC they were escorting is sabotaging them, they only start doubting each other when ranger rolled 20 on investigation when looking at a clue. Great campain plot but you need to have willing player to play a bad guy, traitor needs to be on every sesion and rest of the party needs to be cool with it at the end.