Continuity mistake: When Simon and Jerry are holding Colonel Maguire and Simon says "Look at his hands. Cigarette burns." First Maguire has his hand down, but when it cuts he is holding it up. (00:15:40)
Visible crew/equipment: When we first see Mrs. Paley coming through a door, reflected on the shiny surface of the door is a boom mic. (00:16:35)
Continuity mistake: When Jerry scratches a '5' with his watch onto the wall, in a wide shot he starts to scratch the 5, then in the closeup of him scratching it onto the wall, much more of the 5 has been completed between shots. (00:17:35)
Continuity mistake: When Jerry marks the door with "5" he place his palm flat on the wall. When it cuts, his fingers are bent. (00:17:40)
Continuity mistake: When they start running from the moving wall Jerry grab Sasha holding her left hand. When it cuts he is holding her right hand. (00:20:05)
Revealing mistake: When Kate decides to ditch Maguire and escape herself, we get a shot from above showing her face and Maguire's feet. However we only see his trousers. His feet are gone. Even though some could say that the wall removed them, but we can also see that the wall hasn't reached the part of the feet that are missing. It appears the CGI team forgot to add his feet in. (00:21:35)
Visible crew/equipment: At one point when everyone is escaping through the doors away from the moving wall, when Simon is the last one to climb through the door. When the door closes we can see a persons arm reflected in the door. That is an impossible position for any of the others to stand in, so it has to be a crew members arm. (00:22:15)
Continuity mistake: In one scene Jerry draws a cube with his watch. There's then another shot, then in the next shot of Jerry, the cube drawn is totally different from what he drew a minute ago. (00:26:15)
Other mistake: When Max looks into another room to see Julia for the first time, he starts to enter the room. He is caught off guard by the gravity shift and almost falls into the room, but is able to grab on the rungs before falling. The way the rooms are built, it would be nearly impossible to do this given that the rungs are recessed around the frame of the door. Max would first of all have had to know that the gravity would shift, and then carefully reach both of his hands around the door in an extremely uncomfortable position to be able to touch the rungs, and would even be able to grasp them until he was actually falling. There is just no way that all of this could conceivably occur as it did. (00:29:00)
Continuity mistake: The position of the 'rope' of clothes on which Simon is holding changes. In one shot he is holding onto a particular part of a shirt, but in the next he's holding onto a different part. (00:31:05)
Continuity mistake: When Jerry is telling about the last thing he remembers was going to sleep we can see Simon's jacket hanging on the handles. When it cuts to a close-up of Jerry the jacket is gone. (00:32:30)
Continuity mistake: When they find the dead scientist, Kate helps Sasha put her left foot on the ladder twice. (00:38:05)
Continuity mistake: Kate writes the numbers on her left arm, with the pen she got from the doctor. In one shot though, the writing is on her right arm. (00:39:05 - 00:46:45)
Visible crew/equipment: When Mrs. Paley opens one of the portals, she sees herself getting killed by Simon, then Simon getting his head taken off by something. Then the portal door closes, and a camera and crewman are reflected in the portals shiny surface. (00:42:30)
Revealing mistake: As Sasha stands up, saying she feels something is wrong, to Kate (in the Razorsphere room), you can see smudges and scratches, revealing a protective glass screen between her and the camera. (00:51:30)
Continuity mistake: When everyone climbs out of the 'Razorsphere Room' Sasha cowers in the left corner of the side of the room opposite to where everyones climbing out. But you can see in the wide shot when Jerry gets killed by the Razorsphere, Sasha is now in the right corner of that side of the room. (00:53:05 - 00:53:50)
Continuity mistake: As everyone leaves the Razorsphere room, Julia leaves her high heels on the floor. Later on in the movie we see as she climbs through a portal, she is holding her heels. In the next shot they vanish. (00:53:20 - 01:02:55)
Visible crew/equipment: When Simon shouts to Kate "You're a dead woman!" because she goes back to rescue Sasha from the Razorsphere, the door shuts in his face, and you can clearly see the camera crew reflected in its shiny surface. (00:54:00)
Continuity mistake: When Sasha is trapped in the Razorsphere room, she yells "Kate, help," and then between three shots her necklace changes from tucked under her purple shirt, to hanging out and back to tucked under. (00:54:20)
Continuity mistake: When Simon has Mrs. Paley tied to a ladder rung, prior to the slivers of light coming, Simon has two large sweat patches on his grey tee-shirt. Yet when he climbs out of the portal door, away from the slivers of light, the sweat patches have gone in a matter of seconds. (00:59:45 - 01:01:10)
Chosen answer: I'm inclined to think it was longer than a month because of the lack of proper needed care for such an injury would prolong the healing process. But he only had a handful of souvenirs on him, which means he probably stopped collecting after a while. I concur with your cannibalism theory, there's no other way he would have survived. However that doesn't explain what he could have done for water. This theory of Simon spending possibly months in the cube also brings up the question: If the cube breaks down at 6:06:59, then how is it possible to live in there for months? Well, as we saw, certain rooms have slower and faster rates of time. So, he could have spent all that time in a room with a slow time rate waiting for people to wander into it and feed off of them.