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.
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.
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.
Continuity mistake: During the bat scene the painting on the mantel above the fireplace disappears and reappears, then disappears until the bat is killed.