Deliberate mistake: When the cannister rockets through the ceiling window, the pieces of glass from the initial impact never fall down. Nor are they on top of the window in the next shot. They just sort-of vanish. Additionally, the cracks don't match from the first shot to the second time we see it. There's suddenly less cracks which is odd because it's a digital effect.
Deliberate mistake: During his autopsy in Saw IV, it is revealed John Kramer was 52 at the time of his death. Kramer's role in this film is the very early stages of his time as Jigsaw, shortly after his cancer diagnosis, which would put him in his early 50's. However, he looks far too old to convincingly be that young (Tobin Bell was in his mid-70's at the time of filming). He even looks older than he did when he was on his deathbed in Saw III.