Visible crew/equipment: When the truck flips after the "jar" is blown up, a rig that the truck is on can be seen moving left frame.
Revealing mistake: When Dean shoots the chasing vehicle with a shotgun, it flips several times. Wires are visible pulling the vehicle to the left.
Continuity mistake: The whole sequence after Brill blows up his building features changing skies. At first its partially cloudy, and then sunny with blue sky. Finally when they are going toward the highway, the sun is setting.
Chosen answer: The fake Brill is an undercover federal agent trying to find out what Will Smith knows about the video tape.
Mister Ed
Poor writing though as that character is never spoke of again.