Continuity mistake: During Jigsaw's seizure there is a shot of Lynn when she's yelling at Amanda for help. In this shot she has several strands of hair that are across her face, but in the very next shot they are gone. She couldn't have brushed them away because her hands were on Jigsaw the whole time. (00:37:50)
Continuity mistake: After Lynn looks for pills she runs back over and tells Amanda "Back off, back off", then takes off Jigsaw's oxygen mask and puts her hand on his face to keep his head to the side and you can see two drops of blood on her hand. The shot cuts to a closeup of Jigsaw and now there is no blood on her hand. (00:39:50)
Continuity mistake: The blood spots on Jigsaw's hospital bed disappear during shots of him telling Amanda to let Lynn go, and they reappear in later scenes.
Continuity mistake: The blood spots and streaks on the chained man's body are different in the close up shots of the chains than they are during other shots. (00:06:00)
Continuity mistake: When Jigsaw says "You're already dead on the inside", the oxygen mask is resting on his chest. In the next shot he is holding the mask. (00:25:00)
Continuity mistake: Jeff lays his bloodied hand right in the center of Jigsaw's chest (pointing towards Jigsaw's chin) when he says "I forgive you", but pretty much only his fingertips touch Jigsaw. Right before Jeff slices Jigsaw's neck you can see the handprint, but it's MUCH more defined than it should be AND the fingers are pointing towards the right instead of straight up. (01:42:10)
Continuity mistake: When Jeff is inside the box he kicks out 2 boards, when the shot cuts to the outside there's only one board missing. (00:30:55)
Continuity mistake: When Jeff is screaming at the guy in "The Rack" there are a few spinning shots of "The Rack". You can see that the guy's arms aren't broken, even though you've seen them both be twisted until they snapped. (Slo-mo helps but is NOT required).
Revealing mistake: Notice that throughout the movie the primers (little 'buttons' on the tips of the shotgun shells) on Lynn's neck trap have all been expended. In a sense, none of the shells have the capacity to be fired.
Continuity mistake: When Matthews is smashing his foot with the toilet bowl lid, he probably hits his foot a good six or seven times, and one of the times he also glances against the flashlight and it rolls a bit to the side. The next time you see the flashlight it's pointed right back at his foot like it was in the previous shots. (He was still pounding away on his foot, so he didn't stop and realign it or anything.) (00:03:00)
Continuity mistake: In the close-up when Amanda goes to suffocate Adam with the plastic, the plastic only goes up to his forehead, yet when the scene cuts back (instant cut) the plastic is completely surrounding his entire face and head. (01:16:50)
Continuity mistake: The key is bubbling when it first drops in the acid, and continues bubbling throughout most of the scene. As Jigsaw's message is winding down, the bubbling dies down and stops. But it's bubbling full force again when the detective goes to put her hand in the jar. (00:13:20)
Continuity mistake: The acid is completely red after the woman puts in her hand. The acid jar is then completely clear throughout the rest of the scene. (00:16:50)
Continuity mistake: When Amanda says "I'm standing right here" her hair is hanging down, but brushed back from her right eye, in the previous and subsequent shots her hair is hanging down and covering her right eye. She was holding the gun in her right hand the whole time and all the cuts were instant, there was no time to move her hair.
Deliberate mistake: In Saw III we learn that Amanda was the person who abducted Adam from his apartment. However, if you watch his abduction in the first Saw film, you'll see that the person attacking him has the hands of a middle aged man.
Revealing mistake: When Lynn is inspecting the "reverse bear trap", the trap Amanda was placed in for the first movie, you can see a small orange light reflecting off a few surfaces, like Lynn's arm and the nearby table. This is the brightness level auto-adjustment light that is present on many handheld cameras, which the cameraman apparently forgot to cover or turn off. (00:47:20)
Continuity mistake: The green tube that's connected to Jigsaw is gone when he says "Suffering? You haven't seen anything yet", but it was there in the previous scene and it's back the next time you see him.
Continuity mistake: Lynn explains the procedure she's going to do to Jigsaw, then goes to fill the syringe. Look at the syringe when she sticks it in the bottle, the plunger part is pushed all the way to the bottom. The shot cuts to a different view and the plunger is a good inch or 2 pulled back already (she hadn't started to draw it out yet, you can see 1 or 2 inches of air). The shot cuts again, and now the plunger is at the top of the syringe again, then she starts drawing it out). (01:01:30)
Continuity mistake: When Jigsaw is coughing up blood, the blood in the mask disappears between shots. (00:37:55)
Continuity mistake: Amanda's holding the gun in her right hand in every scene when she's arguing with Jigsaw, except when she's saying "It's all BS," then the gun is in her left hand. It's back in her right hand then very next scene (all instant cuts, no time to switch hands).
Answer: It was too late for Adam to solve the game the way he was intended to- the key went down the drain, which Jigsaw actually didn't intend to happen. Adam's key was meant to be tied around his neck in such a way that neither him nor Lawrence would be able to see, and Adam would be allowed to go free and escape if he were to locate the key- which would only be possible if he looked at himself. The whole idea behind his puzzle was "looking at himself instead of others, for a change." The reason it went down the drain instead, was because of Amanda, who continuously made traps unsolvable. Instead of tying it around him like Jigsaw asked, she just tossed it on his chest, which defeated the whole purpose. Also, he was given a saw just as Lawrence was, as a much more violent plan B, another way he could save his own life.
That doesn't make sense because Jigsaw told Adam that the key is in the bathtub at the end of the movie.