Given what Dipper had markered all over the walls of his MedBay room, Wash really shouldn't be surprised to hear that demonic possession had been a thing where Dipper was from. Shouldn't be.

But, evidently it was, and it's still a surprise. Also, Dipper's sliding scale of weird is completely broken. No question about that.

But somehow - somehow - that's not the point of the story. "As long as nobody was hurt, right?" He settles in and shifts his gaze back to the lake. "So what do you do when your mistakes get people hurt?" That's not a rhetorical question - he wants Dipper's answer.