I have no experience with d20 Cthulhu, so this story comes from the old-fashioned version.
I've never had a game of Cthulhu deteriorate into Scooby Doo, but I have had one start there. The GM made up character sheets for everyone and had us pick them based on what we thought they sounded like, not using their names. There was the driver / leader, the bookworm, the model, the useless character, and the useless character's sidekick. Once those were divided up (we had six players), the last player got his pick of two other random sheets. After everyone had their characters, we were able to turn them over and see what we were playing. It turned out to be Scooby and the gang. The special guest star of this episode was Mr. T. It was a riot.
Everyone played their characters just like Scooby Doo characters would behave, and it was absolutely hystarical. Mr. T. ended up dying after being hit by Scooby and Shaggy trying to crawl into and use an old suit of armor with Scooby acting as the legs and Shaggy as the arms.
Daphne managed to locate an ancient text (everyone knows never trust those things in CoC) but when we were getting beaten by the snow-monster, she read one of the two spells in the book. It was a summoning spell, though she did not realize it at first. She had a choice between a lesser powered version and a greater powered version. Fortunately for everyone, she picked the lesser powered one and we were faced with a SPAWN OF CTHULHU instead of the actual big-daddy.
Daphne and Velma went completely insane after that, but Scooby and Shaggy managed to get to Mr. T.'s van and pull out some heavy artillary (actually, they already had done that to fight the snow-beast, who died the round before Cthulhu's Spawn came to be). Training their weapons, they both fired at the spawn, and all three shots not only hit, they pierced (that's about a 4% chance, in case you're not familiar with the rules). Before the spawn had a chance to do anything, he was killed. It was great.
Unfortunately, Scooby and the gang was no more. Majority of the team was either dead or insane.
If you are a good GM, I recommend trying this kind of adventure. Don't tell your players beforehand what they're playing, and make the character sheets for them. It was one of the funniest roleplaying experiences I've ever had.
-Mike
