In the episode "Too Many Dummies," Jeff Dunham and Darcy Lynn Farmer don't believe that Scooby's a talking dog and think that his talking is just a clever ventriloquism routine done by Shaggy. Nice throwaway gag? Except, if you recall from episodes of the original cartoon, Scooby-Doo, Where Are You!, Shaggy was actually shown to be able to throw his voice to fool villains on a couple of occasions.
"What a Night for a Knight" — Shaggy makes it seem like he's stuck in a pot that's located in a different part of the museum from where he's really hiding.
"What the Hex is Going On?" — Shaggy makes it seem like Scooby's the swami at a fortune teller's shop, although there it was Shaggy's own voice being projected from Scooby-Doo.
"The Cursed Cabinet of Professor Madds Markson!" Penn and Teller are close friends in real life and obviously care a lot about each other—it's quite likely that a big reason why Penn (prior to learning what was really going on) got so scared by Teller's scheme was because he worried about Teller (just like how anybody else would be worried for their friends in a dangerous situation like that). It also prevented him from figuring out the scheme early; if he hadn't been worrying about Teller, he would probably have guessed that Teller was the culprit when he faked an attack on himself and then emerged from the cabinet as the Madds Markson monster. Disappearing/reappearing and transformation acts using cupboards are a standard for magicians, so had Penn been less wound up, his Seen It All attitude would have unraveled the scheme.
It makes sense that some of the guest stars are no longer relevant today (like Urkel). The same was true for The New Scooby-Doo Movies, which mixed black-and-white era comedians with current movie stars.
