When Chet is explaining water skiing to Ben, one shot shows the boat in the background behind the two while they are talking. Although the boat is heard idling, it clearly is not running in this shot (the blower belt is not turning).
When Roman tells Chet about overhearing his insult in the men's room at his wedding Roman states that Chet was talking to his father-in-law. But when Roman comes back to make amends with Chet he tells him that he'd made up the story about Chet and his brother, not his father-in-law, in the john at the wedding.
In the scene where Roman and the girls are sitting in the boat, Katie is wearing a halter but when the boat takes off she is wearing a jacket. In the next shot of the boat she is wearing just the halter again.
When the car is parking inside the four trees at the start of the film the right rear side of the car is an inch or two away from hitting the tree, but when the doors open the tree is a foot or two away.
When Chet and Roman are drinking at the bar the bartender starts pouring a shot of vodka into a rocks glass. When the camera cuts back from Reg, he's making a dark colored mixed drink in a high ball glass.
After Chet starts the speedboat and Roman yells "You got it in Neutral!", Chet pushes the throttle forward. When the boat speeds up the ramp, a cord is visible attached to the boat, possibly pulling it up the ramp.
All indications are the movie is set in Wisconsin yet in the opening scenes as the Jeep is driving en route to the cabin, a deer is seen swimming across a lake but it is clearly a mule deer. Wisconsin has Whitetail Deer but no Mule Deer. The closest wild Mule Deer is probably 1,000 miles away.
Throughout the movie, the characters make reference to being in Wisconsin, but when Chet and the kids go to visit the old man for his 109th birthday, Chet refers to the man as the oldest living person in Canada.
The movie supposedly takes place in either Wisconsin or part of Canada close to the U.S. Midwest which would be Ontario. Grizzly Bears such as the "bald-headed" bear that attacks the cabin towards the end are not found anywhere near those areas.
Roman tells Chet that when he lost everything, he had to surrender his seat on 'the exchange', and that he now wears a blue runner's jacket at work, and fetches coffee. Regardless of which exchange he is referring to (CME or CBOT), runners' jackets are 'gold' (Orange), not blue.
After Chet finishes the Old 96er and everyone is back at the cabin. Chet runs to the bathroom with everyone chasing him. As the camera pans out, you clearly see Dan Akroyd counting down for everyone to scream and run out of the cabin.