LJ's body double... a doll with completely different colored hair is seen in at least two shots. One in the trailer, when Jack is explaining the feeding process and the other, when he is supposed to be sleeping in the crib...
When Bernie is going to fight with Jack, Jack warns him that it would "bring his demise". While Jack is speaking the camera is shooting over his left shoulder and it can be seen that Jack's mouth is not moving.
When Greg and Jack are talking while driving in the camper at night, the shots of Greg all show Jack reflected in the window beside him, and vice versa. In a few of these reflections, the person appears to be speaking when they are not - and Jack is seen speaking into the microphone when he is not.
At the wedding, Bernie mentions having been Greg's father for 36 years. But one major plot line revolves around Greg being 19 when he lost his virginity, and that this happened exactly 15 years ago. If Greg was 19 years old 15 years ago, this would make him 34, not 36.
On the next day after the party, when Jack is sitting in the RV about to leave, Gaylord is shouting at him. While mentioning the injection of the truth serum, he points at the wrong side of his neck. The serum had been injected on the opposite side.
Roz Focker says that "they're probably down in in Little Havana having chimichangas." Little Havana is a renowned Cuban area and traditional Cuban food does not include the Mexican dish of chimichangas.
In Meet the Parents Pam tells Greg that her father had been in the CIA for 34 years. However, in the jail scene towards the end of the movie, Jack tells Bernie that he was in the CIA for only 32 years.
When Mr. Jinx lets Little Jack out of his playpen, the gate is left open. However, in the next shot when Moses the dog is chasing Mr. Jinx, the gate is shut and properly locked. In the next shot, the gate is open again.
When Greg goes into the room where Jack and Dina will be staying, he moves a figurine from the bedside table, hiding it behind the table itself. When they end up going in later, the figure is back where it started.
When Greg is looking throught Little Jack's basket of toys, he holds up the wooden plank with the colored blocks on it, but in the next shot, the block colors are reversed even though he didn't turn the plank around.
When Jack is making the announcement in the RV there is a shot of Greg clapping. In the reflection of the window Jack is speaking into the microphone. Then when finished talking he puts the microphone back into it's place. Jack offers to let Greg honk the horn. When Greg says, "Sure" the reflection of the window shows Jack still speaking into the microphone even though he had set it down.
When Greg is talking to his parents there are two candles to his right by the door. In the first shot both candles are lit, in the second - only the top candle - and in the third they are both lit again.
When Greg and Jack are talking in the RV while on the way to Greg's parents house, Jack makes an announcement on the PA system. After he puts the microphone down, you can still see it in his hand in the reflection in the window behind Greg.
When the whole family is eating supper on the formal dining table, the little Jack is behind the mom and dad of the girl in his playpen. When they change back and forward to Gaylord the kid disappears.
When Jack takes 3 quick photos of Jorge with his camera pen, Jorge is not smiling and he is looking straight ahead. However, when Jack uploads the photos in the email he sends from the command center of the RV, Jorge is smiling in the first photo and has his head turned in the third photo. Also the speed at which the camera takes the photos would be too quick for any physical movement to be made in between.
After Jack gets the massage from Roz, he heads to his camper. After straightening out his back he starts buttoning his shirt as he walks. In the next shot, his shirt is completely buttoned and neatly tucked in his pants. The was no time for him to do all this in the time he was walking.
When Dina has the scrapbook during dinner, she is seen holding the book open to the circumcision page and then flipping through the following pages but in the next shot the book is open back to the circumcision page.
In the RV, Bernie and Roz come into the RV carrying the dog. Jack is holding the cat and then the animals start chasing each other. After they already show them running, the camera goes to Jack and he's still holding the cat.
When Greg is by the awards that Roz and Bernie have displayed for him in the kitchen (Bernie is at the island in the kitchen), the name on the plaque is clearly spelled Gaylorck Focker instead of Gaylord Focker.
Sodium pentathol works as a truth serum in that it relaxes a person and does make them very communicative, sharing thoughts without hesitation. This makes them easily guided in questioning since they speak first and think later. However, the person loses inhibition but not self-control. The person will not simply begin to spill information if they choose not to or are not prompted to do so. Sodium pentathol is also a commonly used anesthetic. A syringe with a highly concentrated dose of that amount would have put Gaylord out cold on the floor. He wouldn't have been awake enough to tell the truth.
Greg is clearly driving the motorhome on the interstate with two divided pairs of lanes going in opposite directions with white (not yellow) lines down the middle, yet one shot shows an SUV going past them in the opposite direction in the lane right next to them (and they're all in the wrong lane, too, for America).
In Judaism, the circumcised foreskin is buried in the ground, yet Greg's parents keep his in a scrapbook. This is more than likely an example of their eccentric behavior, and the fact that they are overbearing and chronicle ridiculous moments of their son's life.
When the camera cuts to a wide shot of Greg and Bernie Focker handcuffed on the cop car right after Jack Byrnes turns around his RV, Greg's handcuffs are open even though he has his hands behind his back.
The goof items below may give away important plot points.
When the minister is playing the wooden flute at the wedding, the pipes he blows are clearly unrelated to the sounds being produced (low notes vs high notes). Also, his mouth moves to other notes on the flute while the previous note continues to be heard.