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.
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.