In the final scene, the car is further into the home than it had been after hitting Denny. It would have had to have been puled back to remove his body, yet it is further forward than it had been.
When Katherine gags Walter, it is a small wrinkled piece of tape. But the shot shortly after shows what appears a bigger less wrinkly piece of tape over his mouth.
Denny tells Jose to watch Walter and Kyle in the living room while he and Frankie go through the house looking for goods. However he left Brooke alone in the car which gave her the opportunity to untie herself and go help the rest. It's unclear why he didn't bring Brooke in the house so she could also be watched by Jose.
Denny's woulds are something out of a bad horror movie with an indestructible boogeyman. Denny was stabbed with a large knife in his back and it does not phase him, whereas his conspirator was stabbed in the foot and nearly incapacitated. Also, there is next to no blood on his t-shirt where he was stabbed. Denny's later wounds include being stabbed in the leg and gut shot by a shotgun, but neither incapacitates him as he is able to attack and fight Katherine near the end.
When Brooke escapes Denny's car, she chooses to go into the house to confront the 3 bad guys herself rather than call the police or for help from a neighbor. A very unrealistic choice.