Barbara says to the radio psychologist that she is 26 years old, while Bette Midler was 41. Dave says he's 27, while Richard Dreyfuss was 39. Both appear too old for the stated ages. However, given the general age of their children they are clearly lying.
Near the end of the film during the pool scene when they've all jumped in, if you look closely you will see Max Whiteman in the pool. But just a a couple of minutes later, he appears on screen with his friends and then jumps into the pool with them.
When the alarm is set off in the beginning of the movie, Max Whiteman jumps out of his bed and grabs his video camera. As he leaps from the bed it is apparent that he is wearing a man's night gown. He runs directly onto the front driveway and appears in long-john underwear.
Jerry loses his dog on Thanksgiving Day morning. Throughout the day he frantically searches along a very busy Rodeo Drive as crowds of people are eating and shopping. However, because it's Thanksgiving, all retail stores and most businesses (except movie theatres) should be closed on that day.
When Dave is returning home following his auto accident, there is a scene with a gardener in the background. He moves as if he is trimming grass around a palm tree, but the trimmer line is not spinning.
When Dave and Jerry are at the beach we see the sun setting behind them, very much toward the northern horizon. As the movie is set around Christmas, the sun should be setting well to the south. It's obvious that the scene was filmed in late Spring or early Summer.