Continuity mistake: When the bully opens Peter's backpack and the mess explodes, Peter has some of the mess in his hair and on his shirt. When the camera goes back to him in the same scene, it's mysteriously gone. (01:01:50)
Continuity mistake: In the lunchroom scene where the bully dumps Peter's chili into his bag; it then flashes to Peter somehow eating his full bowl of chili which was just dumped out. (00:17:11)
Character mistake: After the dodgeball game ends Jerry asks the referee if skyzone had a bar, but in the background he's already walking away.
Continuity mistake: When Peter is in the car, he is wearing a hoodie. When he tells the driver he's only 12, he's only wearing a T-shirt, but as he's getting out of the car, the hoodie reappears. (00:58:41)
Continuity mistake: Grandpa squeezed the mustard bottle, aiming at and hitting the center of Peter's pants where the zipper or "fly" is, plus some mustard landed on his shirt. The stain starts off covering a pretty large area, then shrinks to half the size and moves further right. (01:13:59 - 01:22:59)
Continuity mistake: When Grandpa lifted the lid off the dumpster, it was close to full (all but a few inches to the top). When Grandpa and his friends throw the bully into the dumpster, no garbage could be seen at first, then the garbage was several inches lower than it was. (01:10:40)
Revealing mistake: The boards around the window on the house that got hit with a falling tree fell off separately - individual boards were not connected enough to stay attached. As each board fell, the "wall" around the window could be seen and it was so thin (no thick frame) that it was obvious the house (or that section of the house) was a prop. The roof also seemed far too damaged to be "real" - there was a huge hole in the roof but the falling tree did not have long enough branches to cause that large of a hole. (01:17:20)
Continuity mistake: Grandpa did not get a lot of soda/pop sprayed on his shirt or Santa hat - most went onto his face. However, the "wet area" of his shirt becomes much larger after he left the table. Not too long afterwards, the "wet spot" on his shirt looked much wetter, then soon after he falls down holding onto the ladder it's drier. Also, the white fur around his Santa hat stayed almost dry (or dry) despite being on his head when the soda/ pop (can shaken by Peter) sprayed all over Grandpa's face. (01:15:30 - 01:15:53)
Audio problem: When Peter is sitting next to Grandpa in front of the fireplace, he has several consecutive lines when he's seemingly reading off a cue card, and his mouth movements (or lack thereof) do not match the audio. (01:25:00 - 01:25:21)
Continuity mistake: When Grandpa exits the supermarket and lies on the floor his cap is on. A shot later it lies several centimeters away.
Continuity mistake: When Grandpa is hanging the Xmas lights Sally's arms are crossed. Check her left fingers resting on her right elbow. They change positions in every shot.
Revealing mistake: During the trampoline scene, there's a side shot of Grandpa jumping high. It is a very obvious stuntman for De Niro who looks nothing like him.
Continuity mistake: When Peter reads his essay in class his folder keeps swapping between being open and closed, and parallel or perpendicular to the table.
Revealing mistake: When De Niro/Grandpa falls down the roof and hangs on to the drain, he is replaced by an obvious stuntman with a wig and thinner build.
Continuity mistake: When Grandpa shows his old phone to Sally the way he holds it differs between the front and back shots.
Revealing mistake: Grandpa holds the can of soda/ pop at about a 45° angle when he is going to pull the tab. Robert De Niro, playing Grandpa, is placing the can in the position needed for the carbonated contents to spray all over his face. (01:13:05)
