Continuity mistake: When John Gacy hands a joint to the young guy he's picked up on the corner, the joint is nearly finished. When the young guy hands it back, it's almost a full joint.
Continuity mistake: After Johnny and Mark return to the rooftop following the "Chris R." sequence, we see Johnny run into frame, and Claudette is standing by herself to the side of the others. Mark is not on the roof yet. Immediately after during the next few shots, Mark suddenly appears behind Claudette, and even has his hands on her shoulders.