Clark says he has 25,000 lights on his house. These are the old-fashioned bulbs that draw 7 watts each; thus, Clark would be drawing 175,000 watts of power, or 175 kilowatt-hours (kW-h) continuous draw, far more than can be delivered to a single-family dwelling using the U.S. standard 120-volt electrical system. Therefore, Clark's main breaker would have instantly tripped the second he would have plugged in all the lights at once, not counting the lights on his Christmas tree and everything else electrical in his house.
Clark Sr. says "if one goes out the whole thing doesn't work" about the Christmas lights on the roof. While this is the case for lights on a serial circuit, the lights Clark puts on the roof are on a parallel circuit. If one bulb went out on these lights it would not affect the rest of them.
Two different model years of Ford Taurus wagon are used in the film for the same vehicle. This can be differentiated by examining the interior door trim panels, specifically the door lock, in different scenes.
When Clark is putting up the lights outside, the top section of the ladder gives way and slides to the ground. However, since the lower half of the ladder isn't leaning against anything, there is no way that the ladder could slide this way without falling forward into the house.
When Clark is putting up the Christmas lights on the roof, he tells Rusty to put down the plastic reindeer and to help him. When Rusty drops the reindeer, you can clearly see at least one of their legs break off. Later, when Clark can't get the lights to start, all the legs of every reindeer are intact.
While the family is standing in the doorway after Clark gets the Christmas lights working, you can clearly see that this house is different from the house used for the inside shots. The inside shots show that the wooden stairs are directly in front of the doorway, however when shown in this scene the stairs are gone.
When we see the grandparent's in Rusty's bunk beds, the top bunk is a good four feet or more from the ceiling. But when Clark falls through the attic floor and lands on his feet on the top bunk, the ceiling level is only at his knees.
Ellen is looking at the house when the Christmas lights first go on with the light switch. When the switch is turned off she leaves and Clark returns. The extension cord is not plugged in when Clark picks it up. If that was the case the lights would not have turned on with the light switch.
In the scene where Aunt Bethany hears a squeaking noise, the camera pans to show Uncle Lewis asleep in a chair. Seconds later, when Clark frightens the squirrel out of the tree and everyone reacts to it, you can clearly see Uncle Lewis awake and standing up as the squirrel flies by. After the room clears, showing Aunt Bethany, knitting clueless, and Uncle Lewis still fast asleep in the same chair.
The Griswolds trudge through waist-deep snow to arrive at their Christmas tree, in a geographical area not indicative of the flat landscape of the Midwest, no less, yet when they arrive in the clearing to see their 'Halleluiah' tree, they don't have a single snowflake on their clothing.
When Clark is showing Uncle Louis and Aunt Bethany into the house, you can clearly see an electric cord running from the lighted wreath on the front door down through the mail slot. When the interior side of the door is show, the cord is gone.
When Clark announces he was going to use the bonus check to install a swimming pool (and anything remaining would be used to fly them all out to dedicate it), the camera shot over Ellen's shoulder shows her expression of joy, but when the camera angle switches to show her more face on, her facial expression is more of shock and horror.
When Frances Smith closes the attic door, The steps are clearly on a telescoping rail system. Then when Clark tries to immediately get out he lifts the ladder up and bangs it on the door, showing that it is now the model that folds out with hinges.
When Clark breaks through the plaster ceiling while trapped in the attic he lands on presumably Rusty's bunk bed. The poster of the two turtles, the Bears Pennant, the eagle graphic are all gone, replaced by steer horns and the upper bunk is much closer to the ceiling.
When the squirrel jumps out of the tree, everyone in the room gets up or reacts in some way. The camera then shows Clark yelling for a second before switching back to the family room. When it goes back, some of the people are in different positions then they were before. This is most noticeable with Uncle Lewis who is now standing, Ellen's father Art (in the blue sweater on the couch) because he stands up in both scenes, and Ellen's mother Frances who drops her monopoly cards twice.
When Clark goes outside to cut down a new Christmas tree on Christmas Eve, the lights on the tree that has just burned down (and can be seen lying on the curb, with the lights still on it) are visible through the living room window.
At one point when Snot the Dog is chasing the squirrel through the house, they run over the top of the dining table, and the food, plates, and silverware get demolished. A moment later, Clark is walking past the hallway leading to the kitchen with the table visible in the background. The tablecloth and setting are in pristine condition.
After the ice crashes through the neighbors window the neighbors are looking at the mess. in the long shot looking at the window you can see vases that would have been broken or at least knocked over when the ice came through.
When Clark gets the attention of everyone at the dinner table and asks Bethany to say grace, Clark's mom picks up a glass of wine and holds it in her right hand. When the camera angle changes, her hands are suddenly empty and she picks up the wine glass with her right hand again.
The morning after Clark shoots the neighbor's window out with the ice from the gutter, as Clark is trapped in the attic and the family is getting into the car to go shopping, you can see the window is now undamaged.
The swat team leader tells Clark "I told you to freeze", and Clark asks "May we blink?". Behind the swat leader, the front door is standing wide open. After Clark asks "May we blink?", Mrs. Shirley and the police chief enter the house through the front door, but when they do, they open the door, the same door that was standing wide open seconds before.
Just before Clark takes off down the hill in the sledding scene, there is a shot looking down the hill where you can clearly see the wires/cord in the snow that are used to make it look like the snow flies in the air as the sled races down the hill.
When Uncle Lewis burns down the Christmas tree and Clark sees the ball of fire out of the corner of his eye, he gets up and walks towards the living room. If you look closely, a crew member's shadow can be seen walking past the dining room window behind Clark.
After Clark cuts the Christmas tree and the tree expands to its natural size, the second tree limb to break through the window has a 2x4 attached to it. This must have been used to push the branch through the glass.
When Ellen and Clark are outside alone trying to light the Santa, a "wet-barrel" fire hydrant that cannot possibly be used in Chicago, IL due to freezing is visible. This type of fire hydrant is found in Southern California where there is no chance of freezing.
Clark informs Eddie that the sewer is a storm run-off sewer, implying that the waste material Eddie is pumping into it will not be treated; additionally, with the absence of precipitation or significant melting, Eddie's chemicals remain stagnant until the end of the film. In reality, Chicago has a single, unified sewer system; storm run-off and wastewater run through the same mains for processing by the MWRDGC and the mains are therefore constantly flowing with effluent.
Clark Griswold's Ford Taurus station wagon is shown to have exterior wood paneling, an option that was never available on this car. (However, it's possible the exterior appliques were added by the filmmakers as a nod to the "Wagon Queen Family Truckster" the Griswold family bought in Vacation (1983).)
In the hostage scene, Ellen says to Mrs. Shirley, "This is our family's first kidnapping." Actually, it's their second. They kidnapped the Wally World employee (John Candy) in the original Vacation movie.
When Clark is locked in the attic, he sees a movie reel marked "Christmas 1959", but when he's later watching movies, it says "Christmas 1955". However, there might have been several year's worth of Xmas movies, and the first one we see him watching wasn't necessarily the first reel he looked at in the box.
At the beginning, the Griswolds are driving home with their new Christmas tree, roots and all, yet they brought no tools with them. All four of the Griswold's together could not uproot a 25-foot tall tree, let alone strap it to the roof of their vehicle without tools or equipment.
In any of the varying outdoor scenes, the temperature is obviously supposed to be cold (judging by the snow and ice, the characters' comments, their clothing, etc.) as it normally would be in Chicago in winter. Yet the steam from their breath is never visible as it would be at such a temperature.
When Clark is putting up the Christmas lights, snow covered evergreen trees are visible. When the ladder falls backwards, trees (birch?) are clearly shown with no snow & full green leaves, which they would have lost well before December.
When the SWAT team arrives to rescue Mr. Shirley, the entire Griswold house is dark, yet the whole family is inside with all the lights on, but they are not shining through any of the windows. Even when the SWAT team breaks through the windows they are dark, yet the second they arrive inside the house, it is still well lit.
The attic door that 'traps' Clark in the attic always has a way to open from the inside. The chain on the outside is a simple pull-latch mechanism that can be grasped from the inside; thus, Clark was never really trapped in the attic.
When Clark falls and grabs the gutter, you can see in the gutter Clark grabs has is no snow/ice in it, and that the gutter that Clark is holding onto is broken. Then it cuts to the ladder, the cuts back to Clark holding the gutter, but the gutter isn't broken anymore, and has snow/ice in it.
After they get their first Christmas tree up and Clark gets the sap on his hands, he is laying in bed reading a magazine. Whenever he turns a page, his fingers get stuck in the pages, but when he lays the magazine down, his fingers are not stuck to it, then you can see his hand stuck in his wife's hair and on the lamp.
When Clark and Ellen are in bed after the tree with all the sap, Clark is having trouble with the magazine sticking to him. At the final scene when he turns out the lamp, you can clearly see a handle on the lamp for him to grab to making it look like it was sticking to him. Also, when his hand is supposedly stuck to Ellen's hair, you can clearly see that he's actually holding on to it and pulling it toward him.
After Louis lights his cigar near the Griswold's Christmas tree, there is a ball of fire that rapidly shoots straight out from a gas jet, and is not consistent with the combustion of a dry Christmas tree, which would burn much, much slower, in lieu of the fact that the tree could not have ignited merely from a nearby burning cigar.
During the squirrel chase scene, the dog appears to knock over a cabinet/hutch, but it falls as if it had been pulled out from the middle, not hit by the dog. The dog's body can also be seen going behind the cabinet rather than hitting it.
The goof items below may give away important plot points.
When Rusty resets the circuit breaker after the cat is electrocuted, he simply turns the breaker back on. After a circuit breaker "trips", it would go to the center position, and have to be turned all the way "off" before being turned back on.
When Rusty resets the circuit breaker after the cat is electrocuted the breaker that he resets is a double-pole breaker which may be used if the outlet jumper was removed (2 circuits on 1 device), or the fault current caused by the cat was so great that the panel main tripped before the receptacle circuit breaker tripped.