During the opening trap, at one point one of the five "Bucket Heads" starts banging on the wall behind them. If you look very closely, you can see the set-wall waver slightly as it's struck, revealing it's not really a concrete wall. (And it's not supposed to be the doorway that is later revealed during the final twist. It's a separate section of wall).
"Jigsaw" represents several firsts in the series. This includes the first "Saw" film to have more than one director, the first "Saw" film released more than a year after the previous film and the first "Saw" film to be shot and released in the wider anamorphic 2.35:1 aspect ratio.