Goof (revealing mistake): Chuckie's Saturn shirt is usually a yellow planet with a red ring. In the first season, a common animation error is for the colors to be vice-versa - with the colors sometimes changing between scenes.
Goof (revealing mistake): For the entirety of the diaper commercial flashback, the twins are dressed in the new clothes Betty bought for them. But there is one frame near the beginning of the narrative in the flashback in which they are shown in their "normal" clothes.
Goof (continuity error): In the first season, Chuckie's mother is referenced several times as if she was alive, In the episode "Real or Robots" as Stu tucks Tommy and Chuckie into bed, he even mentions Chuckie's "Mom and Dad" coming to pick him up in the morning. It's later revealed that Chuckie's mother had died sometime before the series started, she is never shown in any episodes, other than in flashbacks.
Goof (continuity error): When Didi puts Tommy in the crib in "Weaning Tommy," when the viewer still sees Didi, the pillow is in Tommy's crib. Then, when the camera zooms in on Tommy, the pillow disappears, only to reappear when the camera angle changes back to where the viewer can see Didi.
Goof (continuity error): The colors of Tommy's shirt and diaper (blue and white, respectively) are inverted when Prudence (The Junk Food Kid) puts her Popsicle down Tommy's diaper.
Goof (factual errors): The owner of the mini golf course tells Stu and Drew they "both lost" when they fail to make a hole-in-one on the final hole. However, since the two of them are playing each other (and each other alone) there is no way they can both lose. The closest this could come to happening is a tie, but a tie is not the same thing as a loss (likewise, it's possible the owner equates failing to get a hole-in-one on the final hole with losing, but it's still the incorrect way of putting it).
Goof (continuity error): While riding Spike towards the sea when trying to return the Sea Monkeys home, Tommy's shorts turn from Purple to Pink in several frames.
Goof (continuity error): Lil's shoes turn blue like Phil's when Chuckie suggests Hubert eats his spinach.
Goof (continuity error): Betty's swimsuit changes from purple to pink in when the parents are introducing the babies to the water. Later her costume changes from Purple to Navy blue between shots.
Goof (continuity error): The shot where Lil sneaks into Howard's weight room at night in her pyjamas at the start is reused when the babies go to take the calculator in the daytime.
Goof (audio/visual unsynchronized): Lil's voice comes out of the Dinosaur version of Phil's mouth in their vision of Reptar.
Goof (revealing mistake): Tommy is on the floor at Grandpa's speech, and reaches up to the table to grab a fork to break off Stu's contraption from his body. There is absolutely no way a one-year-old baby could reach all the way off the ground to the top of a table.
Goof (continuity error): When Josh meets Angelica and the babies, a rear shot shows Phil wearing a diaper as opposed to his blue shorts.
Goof (factual errors): In the closing credits, Tony Jay is incorrectly credited as playing Dr. Lipschitz - he actually plays an unnamed medical doctor.
Goof (plot holes): It is never explained why the effects of the machine that clear up Chuckie's nose stop working after a certain period of time. But if the effects only being temporary is a problem, why can't the doctor just use it on him again?
Goof (continuity error): Phil and Chuckie wear their shorts under Lil's spare dresses but at the park when the two boys fight over Chuckie in the guise of "Chuquita", Chuckie's blue underpants are visible when he falls and Phil is seen to be wearing a diaper.
Goof (continuity error): In "Angelica Orders Out", when Grandpa tells everyone that he taught Tommy how to dance, he shuts the door and then when the next shot comes on Grandpa, the door is open again.
Goof (continuity error): The Babies appear in their Pyajamas when Lou returns from playing cards but several seconds later they are dressed in their regular clothing.
Goof (continuity error): Tommy had to be taught how to scream in "Hiccups" but in "Barbecue Story" he did scream for a long moment.
Goof (errors made by characters, possibly deliberate errors by the filmmakers): In "Johnathan Babysits" right after the babies throw the dirt on Johnathon in Charlotte's office, Phil's shoes are pink like Lil's instead of blue.
Goof (errors made by characters, possibly deliberate errors by the filmmakers): In "Johnathan Babysits", there is a scene when Johnathan puts his hands on his waist. If you look closely one of his hands are backwards. Goof: In the episode "Johnathan Babysits", there is a scene when Johnathan puts his hands on his waist. If you look closely one of his hands are backwards.
Goof (errors made by characters, possibly deliberate errors by the filmmakers): Throughout "The First Cut" Tommy's band aid keeps disappearing and then reappearing.
Goof (audio/visual unsynchronized): When Angelica is telling Tommy that Chuckie was behind the messes and she says, "But he did all that bad stuff you got blamed for," she starts speaking a few seconds before her lips move.
Goof (continuity error): Ickis, Krumm, Oblina and a few other monsters decided to go for chocolate pudding. However in the Real Monsters series it's been stated at least once that monsters can't stand the taste of fresh food.
Goof (continuity error): At a rear angle, Phil's shoe turns pink.
Goof (continuity error): The babies had to ask for a definition of a lie in "Cooking with Susie", but in "Angelica Nose Best" they knew what one was.
Goof (continuity error): Phil is seen wearing a diaper momentarily as opposed to his regular blue shorts in "Big Brother Chuckie".
Goof (continuity error): When Kimi kisses Chuckie after he removes his glasses, she only has two pigtails at a rear angle but when the camera focuses back on her, she has three.
Goof (continuity error): Kimi is seen with a full set of teeth in one scene in "Dil Saver" momentarily.
Goof (continuity error): The Duck on Lil's dress disappears momentarily in the scene after Tommy disposes of Stu's uncomfortable shoes.