The Grim Adventures of Billy & Mandy: Continous Confirmation Bias
Billy thinks Grim killed everyone he cares about. Grim denies his accusation until Mandy comes along with a box and ominously invites them over for dinner. Billy thinks Mandy ate the people he cares about, but then Grim suggests he's jumping to conclusions. The two try to find evidence that she is a cannibal. Billy continues to be paranoid, sticking to his belief that Mandy ate the people he cared about, but Grim keeps a level head until finally joining him in his paranoia, only for it to be revealed that everything was a plan for Billy's surprise birthday party.