The van is shown traveling an extended distance with no one steering, and it never veers from a straight line. Even a perfectly aligned vehicle could not do this due to road surface irregularities.
When the punk is inside the van, the roll cage for the later crash stunt is visible. Next, there is nothing behind the driver seat and the windshield is clear. After the van is rolling down the hill, there is a black screen behind the driver seat and the windshield is tinted to hide the stunt person driving the van. Finally, when the punks are in the van there is no mattress inside, yet when it crashes a mattress is visible to safeguard the stunt person inside.
The punker throws the bass guitar from the roof and hits the leading car's windshield but a moment later both it and the trailing car have broken windshields.
When the 3 punks approach their motorbikes after vandalizing the vehicles, the main punk has a red and white bandanna / handkerchief tied around his right thigh over his blue jeans. It slips down his leg and disappears completely as he hops on and off his motorbike. Yet in the very next scene when they approach the pink vehicle, the bandanna/handkerchief once again appears on his right thigh.