In episode 116 when Justin and Daphne are entering the classroom, Justin has his backpack over his right shoulder. It then alternates between his left and right shoulders. Later it falls off, comes back on, and then is off again.
The Judge when reading the caption on the Indecent exposure case against Vic: She referred to "The People vs. Victor Grassi". In Pennsylvania the plaintiff in a criminal case is referred to as "The Commonwealth".
There is a hexagon shaped window on the wall between Debbie's front door and the stairs that lead upstairs. In season 1 this window was clear and you could see 'daylight' coming through it. In subsequent seasons the glass panes of this window were covered in gay pride colors and had a non-see-through look. When you see the outside porch and front door of Debbie's house, her house is a semi-detached home with the other half of the house attached to the wall that has the window.
The goof item below may give away important plot points.
In the original feature-length pilot episode, Vic tells Justin he needs a card to get into Babylon and gives him his. The man on the doors at Babylon says "date of birth - 1952?". However when Vic dies in Season 4 his tombstone says he was born in 1951.