Revealing mistake: The Gremlin who throws the beaker full of acid has his arm operated by a stick, which is visible as he chucks the beaker. (01:08:05)
Revealing mistake: When Henry Futterman is attacked by the winged Gremlin, most pedestrians just pass by, only glancing occasionally at Dick Miller's frantic performance, since the Gremlin was only inserted. Only when Futterman smashes the pest into the wet cement, the Gremlin is the 'real' thing. (01:02:35)
Revealing mistake: When the female Gremlin shows-up, there's a fuss in the lab and a Gremlin is throwing stuff. The wires on his arm are visible.
Revealing mistake: When the Chinese store is being demolished, a wire is visible on Gizmo's left ear.
Revealing mistake: When Clamp has the Gremlin attached to his chest, his left hand is very obviously puppeting the Gremlin's arm. Clamp's fake arm is very long and makes a weird twisted bend around the Gremlin's back.
Revealing mistake: When the female Gremlin jumps on Forster, his arm is obviously controlling the puppet. His fake arm's awkward position plus a fake chubby hand reveals it.
Revealing mistake: In the kitchen, Crazy Gremlin throws an ice cream at Kate, which doesn't produce a stain and bounces around, revealing it's a rubber prop.
Answer: He wasn't relevant to the plot.
Rob245