When Susan eavesdrops on Julie's and Zach's conversation through the phone extension, she is almost caught when Zach hears Susan's teakettle whistling. With Susan tightly covering the phone's mouthpiece, Zach could not have been able to hear the kettle, since audio only travels out of the earpiece speaker, not into it.
At the school dance the students are all dancing at a rapid pace when the music playing is a slow song, indicative that the music was dubbed in later.
After the scene where Mr and Mrs Rowland speak with Gabrielle, a crew member is reflected on the side of Paul Young's car as he pulls into his driveway with Zach.