Revealing mistake: When they get back from Bunyans and the garbage is on the floor, Chet runs to the bathroom. The family and Roman follow him, you see Dan do a 3 count for everyone to run back In screaming, the blocking of that shot included a gap that you could see them off camera right.
Continuity mistake: During the bear in the cabin scene, after Chet puts the door back up, shortly after, you'll see the door is on the floor while Roman is fighting the bear with the oar from the steps. (Also, note in that same shot, you'll see the bookshelf was destroyed to pieces with its books all over the floor. That scene was never shown and must have been cut from the movie).
Continuity mistake: The scene where John Candy gets the recently rented speed boat stuck in the gates precedes the scene where Dan Ackroyd actually chooses the boat. This is also confirmed as Buck asks his dad if he can go off on his own while the speed boat is being chosen, but in the scene when the boat is stuck only the youngest son is in the back of the car.
Revealing mistake: When John Candy, Dan Aykroyd, and the two boys are fishing, you can tell they are on a studio lot, with a backdrop of a forest behind them. The reflection in the water from the "shoreline" is about 2 feet (at the most) away from their boat. If they were actually in the middle of a lake, the reflection wouldn't be that close, or for that matter, that clear. (01:00:09)
Continuity mistake: When the lodge owner rushes in with the shotgun/lamp to help with the bear, he plugs it in, the light comes on and the shade is in place, then John Candy takes the gun from him. In the next shot when you see him getting ready to shoot the bear from John Candy's view (looking door the barrel of the gun), the light bulb is out and the lamp shade is gone.
Plot hole: At the amusement park when Buck is looking for Cammie, it is lightly raining but no-one seems to be getting wet.
Revealing mistake: When the family pulls into the lodge parking lot and they all open their doors into trees, the tree John Candy hits with his door wobbles noticeably in the overhead shot. From the regular view you can see it's a smallish size tree, but not small enough to move as much as it does, regardless of how hard he opened the door into it.
Continuity mistake: In the scene where Chet is packing up to leave after the skiing incident, and his wife Connie is attempting to talk him out of it, you can see her necklace, in various shots, under her shirt then out again several times.
Factual error: When John Candy throws alcohol on the fire after his bear story, there is a big explosion, but no amount of alcohol could cause an explosion big enough for a mushroom cloud, only gas or lighter fluid (especially the only small amount that is in John Candy's glass he's holding the glass is barely full and mostly ice.).
Continuity mistake: During the bat scene the painting on the mantel above the fireplace disappears and reappears, then disappears until the bat is killed.