In the scene where Bob is videotaping the hotel room, he zooms in on Gail's stomach, which is very large and round, expecting a child. In the next scene at the wedding reception, it shows them dancing around, and her standing there clapping, and there is no pregnancy belly.
When Bob is at the amusement park & walking down the ramp to get on the roller coaster (which is really Colossus at Six Flags Magic Mountain in Valencia, CA), it's obvious he is going the wrong way down the exit ramp rather than using the real entrance. In the next shot, he is shown on the other side of the roller coaster track getting into the coaster.
When they are leaving the airport in Detroit, to go back home, there is clearly a palm tree in the background, just outside the terminal. Palm trees don't grow in Michigan.
During the dance scene at the wedding reception in the very beginning, one of the extras (who dances with a girl in a green dress) looks directly at the camera. At first you think he's looking at Bob but Bob is on the other side of the dancing and not focused on the extra.