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Incorrectly regarded as goofs
Mr. Peltzer tells Billy one of the rules is to keep the gremlin away from water. He tells him, "Don't give him any water to drink and whatever you do, don't give him a bath". We learn later in the film that they multiply with water. Yet they can still drink beer and soft drinks (water based) without any signs of multiplication. However one reason might be that Gremlins and Mogwai can only multiply if they get wet externally not internally.
Another reason might be that they can only multiply from coming in contact with normal water anything like Beer or Soda may not trigger their multiplying ability.
After Billy accidentally squirts the shaving cream at his father's face, Mr. Peltzer tries to clean himself off. In the next shot, his face is completely clean, but his chest is covered in it.
When the Gremlin is hiding under the table, the plastic brain over the table changes position between shots.
As Grandfather leaves the house after retrieving Gizmo, he is holding the "smokeless ashtray" gift in his hand as he walks out the door. In the outside shot immediately following, the ashtray has disappeared.
When Mr. Hanson is looking for the changed gremlin after the class leaves, the movie projector is still running and gets to the end of the reel. You can see the film flapping as you would expect, but it's on the feed reel, not on the take up reel on the back as it should be when a film finishes. Not to mention there is no drive on the feed reel, only the take up reel. (The feed reel just turns as film is pulled from it.)
The holes that Stripe puts in Billy's sweater at the school are different at the end of the film from when they are in the department store.
A Volkswagen Beetle does not blow steam after repeated starting attempts - air cooled VWs do not have radiators.
Early in the movie one sees the snowy town, yet the snow on vehicles is inconsistent and is clearly sprayed onto the cars. Also, there is no snow or water on the floor of the bank.
The Gremlins run part of Snow White and the Seven Dwarfs (1937), starting with "Heigh Ho", which is at a point much closer to the start of the movie than the "reel 4" indicated on the header would include.
Snow on the underside of branches.
Pete Fountaine uses metal scissors to cut the Christmas lights a Gremlin is hanging off of. Not only are there no sparks from the live wire when this occurs but both sides remain lit despite the circuit's being broken.
Mr. Peltzer tells Billy one of the rules is to keep the gremlin away from water. He tells him, "Don't give him any water to drink and whatever you do, don't give him a bath". We learn later in the film that they multiply with water. Yet they can still drink beer and soft drinks (water based) without any signs of multiplication. However one reason might be that Gremlins and Mogwai can only multiply if they get wet externally not internally.
Another reason might be that they can only multiply from coming in contact with normal water anything like Beer or Soda may not trigger their multiplying ability.
When Billy puts his name plate on the counter it is upside down but when Mrs. Deagle shows up to complain about what Barney did to her snowman the name plate is correctly right side up. A considerable amount of time passes between Billy's arrival at the bank and Mrs. Deagle's. A deleted scene shows that Kate fixed it.
Gizmo is meant to multiply when getting wet. However when Barney the dog licks him nothing happens and there is hardly even a flinch.
However, it is stated that they multiply with water, never saliva.
Gremlins aren't allowed to come in contact with water. Stripe hides in the snow while watching a man reach into a mailbox. While snow is frozen water and that could be the reason he is supposed to be able to do this, the moment he even slightly warms up he will now have water on him. This would have been explained in Colombus's original script, which included this line for Roy, "They can't multiply by water if their environment temperature is below 25 degrees".
Right before Mr. Wing arrived to take Gizmo away, Lynn says she's gonna make Gizmo chicken soup, but water is the main ingredient in soup which causes Gizmo to multiply which is against one of the rules. However, there are numerous other ingredients in the soup; it is probable that only fresh water makes Gizmo multiply.
The items that the gremlins use, such as guns, tools and utensils are too small to be normal (human) sized and appear to be made specifically for gremlins.
When Billy walks through the town on his way to work, even though there is heavy snow in the town square, the hills behind the cinema have no snow on them.
In the department store, Stripe attacks Billy with a chainsaw. Billy uses a wooden baseball bat to try and fend him off. The chainsaw would have cut through the bat in no time, but is shown chipping away at the bat for some time.
When Billy goes in to the Y.M.C.A "Stripe" jumps out at Billy and Billy lands on the floor and the tear marks are already on his shirt before "Stripe" claws his chest.
After the gremlin has tampered with the Sheriff's brakes, the police car nevertheless slows down, displaying its braking lights, as it approaches the next corner. In the next shot, the brakes are not working again. Also, the gremlin is seen ripping off the front hydraulic brake line - any automobile manufactured after 1967 has a dual circuit master cylinder (as mandated by the U.S. Department of Transportation) where one hydraulic brake circuit remains operational when another fails.
As Mr Hanson inserts the hypodermic into the mogwais paw the blood spurts into the syringe behind the plunger before the plunger is pulled to withdraw the blood.
At the beginning of the movie when it shows outside of Billy's house, there's a Santa blow mold that at one point in the movie it is on steady. Then at the end it shows that it is blinking.
When Mr. Hanson is calling the gremlin to come out from under the desk, his mouth does not move with the words being said.
When the cocoons are opening up in Zach's room, you can clearly see a green light bulb inside one of them.
When the police car is shown upside down, you can see the steel skid plate on the underside of the car that was used to make it jump the truck.
When Mrs Deagle is walking down the street with the snowman head and is walking in front of the video rental place, you can see the crew reflection in the window.
As Billy's mother attempts to back another gremlin into the microwave at one point, when things cut to showing us the scene from the top of the oven POV, you can see the puppeteer's head ever so briefly before he ducks below the obscuring microwave.
The arm of a puppeteer can be seen when the gremlin is attempting to choke Billy's mother.
For a movie supposedly set in late December in a Northern State, all of the Deciduous trees still have leaves on them. The most obvious place to see this is in the opening scene when Billy is trying to start his car in front of his house.
In the bar, Kate notices that gremlins are afraid of bright lights when she tries to light the cigarettes of one with a match and the gremlin repeatedly shies back. Yet the whole bar is filled with wafting smoke - supposedly from lit cigarettes and cigars, which we can see gremlins smoking throughout the scene.