Continuity mistake: When the Aunt flags down the cop car, the cop has his seat belt on, as the plant starts coming in the car to grab him and the car. He jumps out of the car without ever undoing his seat belt.
Factual error: When it's 1968 we see a boy (I'm not sure which one so you'll have to look) riding a Shwinn Sting-Ray Krate Grey Ghost. That bike didn't come out until 1971 and they only made them that one year.
Continuity mistake: When the insect sits on the windowshield of the car Robin Williams says that the insects can't make a hole, they are safe, then the insect makes a hole, the car drives away creating havoc on the road because of extreme bad driving. The car stops, they get out, then there is no hole in the windowshield.
Continuity mistake: Carl's cop car has the licence plate "G2298" which is printed thinly on the licence plate. Then, when the vine wraps around his car, and bends it in two, the licence plate is now "CFJB95", printed very boldly on the plate. The words "POLICE 004" also differ, as does the bumper and the lights. Totally new car, methinks...
Revealing mistake: During the monsoon scene, everyone is on a table trying to climb on the chandelier. Peter has been turned into a monkey, and his face is covered in hair and has the characterists of a monkeys face, yet in shots when they are on the table, like when the crocodile snaps at them on the table, just before Alan lifts Peter onto the chandelier, you can see the plain face of his stunt double, with no monkey features! Robin William's and Bonnie Hunt's doubles are rather obvious in these same shots too.
Continuity mistake: When Peter runs out of the attic to the woodshed to get the axe, to the left of the attic door is a large filing cabinet, with nothing in front of it or beside it. Yet later when Peter is swinging the axe at the spiders, and following shots, a green suitcase has appeared in front if it.
Visible crew/equipment: In the scene when the pods and vines start growing all over the lounge, there is one shot of a green vine crawling towards a carpet, and then slithering under. Watch closely, near the carpet, and you can see the wire attached to, and controlling the vine, because the light shines on it, exposing it rather well for a second.
Continuity mistake: When Sarah, Judy and Peter are in the store, where the riot is going on, they spot the Jumanji Board Game sitting on the till counter. To the right of the till is a very large "Sand and Water Activity Table" box. In one shot the box it facing so you can see the upper part/top of the box. It then cuts, and the box has mysteriously rotated 180°, so that you can now see the lower part/bottom of the box! In the same shots, there is a trolley by the till, with some items and a box in it. Cut back to the trolley and there's now two boxes in it! It then cuts, and the box is back to it's original position, and the box in the trolley disappears! If you look more closely, the entire shelf unit behind the counter is completely different!!!
Continuity mistake: When Peter rolls the dice prior to letting the monkeys loose, whilst him and Judy are in the attic, he rolls both "1"'s. The sides of the dice which are visible read "3" on one die and "4" on the other. Then when it cuts to a closeup of the boardpiece moving, the "4" now reads a "2", and the dice have also moved position.
Continuity mistake: When the monkeys are causing chaos in the kitchen, a monkey throws three knives at the kids. Two of them stay in the door, but one bounces off and only makes a nick in the wood. Later in the film, when Robin Williams is in the kitchen, and he blocks the door, you can see that the two knives that were stuck in the door are still there, but the nick in the wood has gone.