At about 1:04:00 in, Brandi is chasing Rachel outside the cabin holding the gun in her right hand.
Brandi shoots once at the fleeing Rachel and hits a tree.
When the film cuts back to Brandi, she's holding the gun in her left hand. The film follows Rachel in the glades when she thinks of the ring ploy. The next shot shows Brandi holding the gun in her right hand (albeit cupped with her left hand).
Right after the wedding kiss, when Brandy drives off in anger, a close-up of her sitting in the yellow Ferrari as she is taking off, shows the steering wheel actually hit the key-off lock right as she starts to turn it. So the car she's sitting in for the shot isn't really running nor is the key on.
When Brandi shoots Carson, there is blood shown on Rachel's shirt. You can tell the blood is fake because it is pinkish and not spattered. Real blood is much darker.
The latex gloves that Rachel wears are too big for her hands.
During the close-up of Brandi taking off in the yellow Ferrari at the wedding, the sound is of a running sports car motor being put in gear and taking off, but in the close-up the steering wheel hits the lock just as she starts to turn it, so the key must be off and the motor can't actually be running.
After Brandi shoots Carson in the head, Carson is shown lying on the ground and blood slowly comes out of his head, in reality, blood would have came spraying out fast.
When Brandi shoots Carson, she is not wearing latex gloves, so her fingerprints would be on the gun.