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.
Continuity mistake: In the scene where the van crashes into the dock's factory doors, the van is stood there in the factory. But when it cuts to outside of the factory, the van is on the road, and no-one drove it there.
Answer: It's never explained. We're just left to assume he was somehow able to.