The needle pit scene was done with real needles. The crew had to remove the tips and add a fibre optic tip. The pit was filled with slime to make the needles slide around more. 120,000 needles were used during this scene. This from the DVD extras.

During the beginning scene with Michael being tested you can see a close up of his eye and the knife very close. This was not actually filmed with the actor, it is one of the assistant camera men.

In Jigsaws lair, where he was caught, you can see items around the room from the house in which the 8 victims were being held. You can see the oar Amanda picks up from the basement and the X marks the spot photograph. These are clues to show that the camera footage is recorded. This is confirmed on the DVD commentary.

During the opening scene Jigsaw shows the head-trap victim where the key is. Jigsaw is seen leaning over a table with the victim lying unconscious on it. This wasn't actually the actor who played Jigsaw; it was a cameo appearance from director Darren Lynn Bousman.

During the scene where Detective Matthews is taking to Kerry in his office, you can see a mugshot of a man with an afro. This is the director Darren Lynn Bousman.

The bathroom where Adam and Zep were found dead was a replica of the bathroom used in the first film. They re-created it from scratch by using movie stills from Saw 1. Confirmed on the Saw 2 DVD special features.

In the room with the razor blade box, if you look at the top of the frame, you can see a lock with a key in it. If the actress walked a few feet further, she could've unlocked the box and gotten the syringe. Obviously this would've defeated the purpose of her dying, but interesting to point out. Confirmed in DVD special features.