Walter finds one of two missing bullets after concluding that he (and his client) had incorrectly guessed the location of the shooter because of an echo, substantially changing the bullet's trajectory (in fact, nearly reversing its direction). However, he operates for the rest of the investigation seeking the second bullet on the (correct) belief that his client successfully hit the shooter. If his client was so incorrect in his guess as to the shooter's position as to so substantially change the trajectory of the bullet, there should be no way that he could accurately hit a moving target, at night, with a pistol, within three seconds as stated.
During the hypnotism/dream sequence where Walter finds the stolen Humvee there is no sight attached to his rife, it has a fore sight because that is fixed but you can clearly see there's is no rear sight.
Leo makes reference to bringing Ross back to life after his execution so he can be electrocuted again. Florida executes by lethal injection. It still has the electric chair, but it has to be requested by the condemned.