Continuity mistake: When Spencer is talking to his grandfather about Martha before going to sleep, Spencer has his right hand on his chest close to his neck. When the camera cuts to an open shot, both arms are down close to his sides, and then when it cuts back to a closeup, the hand is back again on his chest.
Continuity mistake: When the characters are dropped from the plane. Ruby Roundhouse fell first and Shelley fell out last. However, when Shelley gets up after the fall he is standing between Ruby and Dr Bravestone. Ruby has to walk past Shelley to get to Dr Bravestone although she should have fallen close to him, but very far from Shelley. The plane also could not turn in such a manner to drop them next to each other. Shelley held on for a while, so should have been dropped very far from the others. (00:28:15 - 00:29:20)
Continuity mistake: Shortly after arriving in Jumanji, Nigel flies over in the airplane. They all follow Ruby towards the clearing where the plane is landing. When they emerge from the jungle and stop just outside the tree line, we see them from the front, and they are standing in knee-high vegetation. The shot changes to show all of them from behind as the plane pulls up in front of them, only now the vegetation is very low cut, down around the top of their shoes. The shot changes again to show the group once more from the front, and they are again standing in knee-high vegetation. Furthermore, when the plane pulls up, the group is standing off the left wing of the plane. Shortly before take-off, we see an overhead shot of the clearing. The group is still standing off the left wing tip, in the low cut vegetation, while off the right wing tip you can see taller vegetation of the sort the group was standing in when they first exited the jungle into the clearing.