Visible crew/equipment: When Shaggy and Scooby are riding the quad bikes, they stop to pick up Mary-Jane. They then ride off, and one man jumps athletically into the air, to try and catch them, revealing knee-pads under his shorts to protect him when he falls to the floor. (00:48:10)
Revealing mistake: When Fred smashes his arm through the round window of the garage and grabs Shaggy, if you look you can see some kind of clear tape on his arm to protect him from the glass. This is quite noticeable in closeups. (00:47:10)
Answer: It's never explained. We're just left to assume he was somehow able to.