Visible crew/equipment: During the film, set lights and reflectors can been seen reflected in Daphne's sunglasses and Velma's glasses numerous times, especially in closeups. (00:16:15)
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)
Answer: It's never explained. We're just left to assume he was somehow able to.