Continuity mistake: In the trailer, there is a scene showing Matt Damon in a fight. The other person pulls out a knife and cuts Damon on the right side of his face, leaving a big scar. In the next scene of the same commercial the scar changes from the right side of his face to the left side and then back again.
Continuity mistake: After the breaking-in job John gets off a white horse that stands still when he walks off. In the next shot the same horse comes trotting from the direction where John is heading. When John reaches the gate the horse stands again at the same place where John left it.
Revealing mistake: When Alejandra comes to John's room the first time you see distinct t-shirt marks on John's upper body, his arms as tanned as his neck and face. This in spite of the fact that he wears long-sleeved shirts throughout the movie (except, of course, during his nightly encounters with Alejandra).