Continuity mistake: Not only does the key in the acid stop bubbling, then start up again as Kerry's reaching into the jar (as mentioned in a previous mistake), but in almost every shot the key switches from bubbles to no bubbles. It also moves position from the back of the jar to the front of the jar. (00:13:15)
Deliberate mistake: The scenes where the left, then the right foot is being twisted are the same shot, just flipped. You can tell because the blood on the foot and the blood on the machine are in the exact same locations in both shots.
Revealing mistake: After Tony rips his right hand from the chain you can see that the wound doesn't go all the way down to between his fingers, it stop about 1/2 an inch from the webbing of the fingers. The only way to get the chain out oh his hand was to completely rip it out between his fingers, the wound should have gone all the way out. (somewhat difficult to see, but just look carefully [slow-mo helps, but it NOT required to see this mistake]). (00:08:40)
Continuity mistake: When Jeff comes in to his daughter's room the blanket that she has around her chin changes position from shot to shot. It goes from right next to her chin, the a few inches below it, all the way down to spread out on her bed. She doesn't change position in any of the shots, so she couldn't have pulled it off herself. (00:32:00)
Continuity mistake: In the first shot of Kerry watching the Jigsaw tape you can see 2 folders to her left. She fast forwards the tape a bit, the shot cuts back to her, and one folder is inside of the other. Also, all the pictures spread out in front of her change positions and placement drastically. (00:12:00)
Visible crew/equipment: When Jeff is stuck to the fridge, you can see a cameraman's shadow on his shirt.
Continuity mistake: Amanda's sweat marks and eyeliner runs constantly change shape and size (bigger, smaller, and back to bigger) during the bathroom flashback. (01:10:30)
Continuity mistake: The clamps on Jigsaw's head change their positions between the time Lynn says "He's stabilized" and "I need you to raise your hand and spread your fingers" The top one is most noticeable, it goes from hanging over his forehead to lying on top of his head.
Continuity mistake: The blood splatters on Lynn's face change throughout the whole brain surgery scene. The splatters go from defined, to smeared, and then back to defined again a couple different times. (01:03:15)
Continuity mistake: In one shot, when Kerry is in the 'Angel' trap, her right hand is on the chain. The shot cuts away to an over-her-shoulder view and her hand is off the chain. The shot cuts back to show her and her hand is back on the chain. (00:15:10)
Continuity mistake: After the scene of Tony there is a shot of Kerry and Rigg, the detective is in the background inspecting a piece of the chain. The shot moves to the background detective for a second and he looks up at something. The shot instantly cuts back to Kerry and Rigg and the background detective is suddenly looking down at the chain again. (00:10:25)
Continuity mistake: The positioning of Amanda's green sheet changes drastically during her cutting scene.
Continuity mistake: The red cloth (bag?) that's covering the gun changes its position DRASTICALLY between the first time you see it (then he talks to his daughter when she comes in the room) and then the second time you see it. The first time the back half of it is folded back over itself and it's almost tucked under Jeff's leg. The next time you see it it's completely spread out flat. Jeff didn't rearrange it while he talked to it daughter, it just "magically" happened. (00:33:00 - 00:33:55)
Continuity mistake: While Jeff is contemplating burning his son's possessions his hair constantly changes from messed up and down, to pushed up and away from his face. One of the shots you can see Jeff put his hand to his forehead in agony, so this explains that part of the hair change, but the changes happen several times during the same scene.
Continuity mistake: The length of the cut on Jigsaw's neck changes constantly throughout the final scenes.
Continuity mistake: Jigsaw gets "blood" all over the top and sides of the lid of phosphorus paint, yet there is hardly any "blood" on it when he throws it in his bag.
Continuity mistake: When Jigsaw is in the process of passing out (after surgery) Lynn's hands move off the bed, back on, then off again, then back on (it wasn't just her moving them naturally, they were in the EXACT same positions each time they were back on the bed). (01:04:00)
Continuity mistake: When Lynn cuts open Jigsaws head, she clips off some veins with clamps and lets one fall in front of his face. A scene change to the front of his head shows no such clamp lying across his face. (01:03:05)
Continuity mistake: When Amanda takes some items out of her box, she lays a red cloth on the bed and then places some items on it. She places a razor blade down on the cloth and there is a large crease underneath the blade. Cut to another shot and the cloth is completely smooth; no crease. (01:07:50)
Continuity mistake: Matthews is in the bathroom and gets the saw, he shines the light over and sees the severed foot on the other side of the room. He shines the light down to his foot, then he holds the flashlight in his mouth while he yanks his sock off. The next shot (which is a direct cut, you can tell by his sobs) has the flashlight suddenly on the floor and already pointing at his foot while he's stuffing the sock in his mouth to muffle his screams. (00:02:00)
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.