Continuity mistake: When Monster is reading the love letter to Rudy in the trailer of the semi, when Monster opens it up initially, the letter is folded in fourths (halves down the middles), when they come back to him in the shot, the letter is folder into a tri-fold (like a normal letter would be folded).
Continuity mistake: Watch for the part where Gary Sinise and Charlize Theron are in the pool. (All the attention would be focused on her when she removes her top.) But watch when she flings the top to her right, it lands so nicely on the hand rail. As she moves towards Gary in a shot you will see the top lying on the deck of the pool.
Continuity mistake: When Gabriel is using Rudy as a human dartboard he throws two darts in him, one in the chest, and one in the shoulder. A few seconds later Rudy slumps forward and there is a dart in his back, which is impossible since he was sitting with his back against the wall.
Plot hole: When Rudy removes the leg of the bed to escape from the motel room, he finds Charlize Theron and Gary Sinise in the pool. He walks away saying something to the effect of "I am out of here." As he trys to escape, he keeps running into the other members of the gang and has to rush back and pretend he is still handcuffed to the bed when they check on him. If he is already free of the bed, why doesn't he leave after the gang goes to sleep for the night?