When Wally first points the revolver at Lori, you hear a sound as though he is cocking the hammer, but the revolver he is holding is a hammer-less model. There is no hammer to cock and make this sound.
The goof item below may give away important plot points.
When Lori and Wally find the dead body of the real Spencer, Wally throws something at him to see if he will catch it. The object lands in his lap, yet in the next shot it is gone. The object then reappears in another shot.