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).
The film is edited by Kevin Greutert, who edited "Saw" 1-5, and directed the sixth and seventh films. Greutert is one of only a few holdover crewmembers from the original run of the series to return. He said he felt an obligation to be a part of "Jigsaw"- feeling that given his past with the series, he should help usher the franchise into the new direction that the producers and writers wanted to take, while also helping to maintain ties to what came before.