Stupidity: The villain should've removed the tape hidden in the mask once he had it as it was incriminating evidence.
Bedlam in the Big Top - S1-E10
Stupidity: The gang drives away from the circus in the Mystery Machine, and none of them notice that Scooby-Doo, a large, talking, eating-machine dog, is not with them until after they had been on the road for at least several minutes.
Chosen answer: A ghost wouldn't have to keep any food in the fridge - they don't need to eat. So it is one thing that proves the ghost is not a ghost.