This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Big Top Scooby-Doo!
The flag with the clown head on it that Scooby and Shaggy encounter while they’re searching around the circus.
The Scooby in Shaggy's dream wears a very disturbing Slasher Smile while performing its feats of wonder.
A healthy dose of Fridge Horror makes one realize how dangerous Archambaut really is. Most of the newer movies (Abracadabra-Doo and onward) have the people behind the hoaxes openly admit they were never trying to hurt anyone. While Doublebay and Archambaut's werewolf scheme didn't hurt anyone either, when it falls through, Archambaut decides to just straight-up murder Marius in his trailer. Think I'm exaggerating? Remember that Archambaut was slamming Marius against the wall by his neck when Scooby and Shaggy looked in. You can see his thumb pressing into Marius' throat. And Archambaut is a real strongman, so he knows he could really hurt Marius if he got angry enough. Sure, he tied Marius up as he stole the box office money, but he probably wasn't going to leave any witnesses, given how quick he turned on Doubleday, not with his strength and a dart gun full of super-strong animal tranquilizers. If Velma hadn't realized Archambaut's involvement, he very well could have committed murder.
And remember, this was someone Marius worked with every day for at least a year. Someone who was angry and jealous of him for inheriting the circus, and was strong enough to snap even a fit guy like him like a twig. Teaming up with Doubleday was probably the only reason Archambaut didn't try going after him sooner.