When Derek (Steven Yeun) and Melanie (Samara Weaving) gain entry into "The Reapers" office, a nail is already lodged in his desk. A few moments later, Melanie is seen shooting that very same nail into the desk, which barely misses the Reapers' hand.
At minute 54, the girl puts on her pink sunglasses when she already had them on.
Nearly all modern nail guns have a safety mechanism that prevents them from being fired they way they are in the movie. You must push them up against a flat object for them to fire a nail.