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)
Deliberate mistake: The elevator operated by gremlins starts ascending at around 5 stories per second, based on the floor readings. Yet when the elevator suddenly stops dead, Kate does not fly into the air caused by the huge speed change. (00:50:05)
Continuity mistake: When Billy and Kate return to the Clamp building as the Gremlins hatch, look closely - Kate is wearing beige/tan colored pantyhose/tights in the first scene of them running to the building. ("How long do they take to hatch?", "Not long enough, come on!") Almost immediately after arriving, Kate is told to begin her duties as a tour-guide, and thus doesn't have time to change her outfit. However, in one of her next scenes, she is suddenly wearing brighter cream-colored pantyhose/tights. (And given her fairly consistent screen time as a tour-guide, it's clear she didn't have time to change this one part of her outfit).
Trivia: When that insane gremlin has Billy sitting in a dentist chair and is attacking him with the dentist tool, at some point he asks, "Is it safe?", for many it must have seemed senseless, but it is actually a nod to the 1976 movie "Marathon man", in which, during a scene, Dustin Hoffman is sitting on a dentist chair while tortured with a dentist tool by an ex- Nazi (played majestically by Sir Lawrence Olivier) who keeps asking him the same question once and again.