When you watch a film, do you notice which hand the character on screen is holding the gun in?  And if you do, would you notice if in a following scene, the gun happened to be in the wrong hand?  I guarantee you anyone who has ever worked on editing a film will notice this.  They can't help it.  When shooting the same scene over multiple days (which happens often), consistency is key.  It's usually even someone's job to check this.  The problem with noticing the gun is in the wrong hand though, is that it overshadows your experience of the film.