Deliberate mistake: Lily was an infant for all of season 5, but beginning with this season, she is around 5 years old. The show didn't jump forward in time, because JT states in a season 5 episode that he is 19, and in the episode this season where he and Rich go to Mexico, he states that he is almost 20, as well as Al and Mark are both still in high school.
Factual error: Carol and Frank leave for a romantic weekend and Dana begins to tutor Rich for a test in English Literature. When Frank and Carol return on Sunday Rich has not only studied for the test but also TAKEN the test and gotten the results back. How is that possible over a weekend?
Factual error: When they do the opening credits with all the characters, it shows them at an amusement park. It is Six Flags Magic Mountain. At the very end of the opening, they show Six Flags Magic Mountain as if it were next to a beach. You can tell the water was computerized since Six Flags is no where near the ocean.
Continuity mistake: The layout of Frank and Carol's house is inconsistent with the exterior shots. The driveway as seen from their backyard is more shallow and narrow than what is shown in the exterior shots, and the stairway to the left of their front door has a window halfway up, but the exterior shot shows a low section of roof that would completely obscure any window.
Audio problem: When Cody is playing the blues on his guitar, he is shown to be fretting the higher pitched strings while playing the lower strings open. However only low notes are heard and at least three different chords are heard, which would not be possible if he was playing the strings open.
Character mistake: Mark walks into the house and is surprised to see Cody there, and remarks that he heard he was back. However, just before this, when Mark entered the house, Cody's van - which Mark had to walk by to enter the house - was visible through the open door, so that should have confirmed it for Mark before he saw Cody in person.