Revealing mistake: When Becky meets Simon in the cube, they hug, then Simon literally stabs Becky in the back. As she falls down to the floor, her black shirt reveals not only some of her chest, but part of a black microphone wire. (01:15:15)
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)
Revealing mistake: When Colonel Maguire is hitting the walls with his suitcase, there is a shot where the suitcase is coming towards the camera. Dust and stains reveal the protective glass that is in front of the camera. (00:06:10)
Revealing mistake: When Becky climbs through the door and falls upwards a lump appears on her back revealing where the wires are attached as they pull her up. (00:02:40)
Revealing mistake: When Kate opens one of the doors to find Simon on the other side ("Hello gorgeous, miss me? Huh? Come on in, the water's fine."), the shot of her climbing up to open the door is a recycled shot from 42 seconds earlier. It's pretty obvious as the shot still contains a mistake from when the shot was used previously - a boom mic is reflected in the door's shiny surface as Kate climbs up to open it. (01:18:40)
Revealing mistake: When Kate is in one cube room on her own, looking around, just after losing Simon and just before meeting Sasha, as the camera pans around the room, on most of the black cube doors, clearly reflected on the shiny surface of them, are camera / stage lights. (00:10:15)
Revealing mistake: After Kate kills Simon after he kills Sasha, the small hands on the four watches that he is wearing move at different times to each other when they are supposed to be synchronised.
Revealing mistake: The way Jerry comes into play is problematic. He is shown coming up from the bottom of the room where Kate has just met Sasha. He appears to be climbing upwards, which suggests that the gravity in the room that he is leaving is the same as the one that he is entering. However, the way that he positions his body around the square opening doesn't make sense. He turns around to see the girls. At this point, it would be very difficult for him to stay up, because he no longer has support like he did when he first came up. This shows that the actor was standing on something.
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)
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.