Other mistake: When Scrappy pees on Daphne, you see Fred start yelling at Scrappy, but if you look at Fred's eyes, they are focusing on something else.
Other mistake: When the gang is in the mystery machine and Scrappy pees on Daphne and they kick Scrappy out of the car, from a distance the driver isn't Fred - it is someone with dark hair, not blonde.
Other mistake: The scene in the end where Shaggy and Scooby are daring each other to eat hot peppers, Shaggy puts a pepper on a slice of pizza and eats it. Look carefully, he actually doesn't eat the hot pepper on the pizza slice, but still somehow gets the hot taste.
Answer: It's never explained. We're just left to assume he was somehow able to.