After Ethel knocks the delivery boy unconscious with a glass bottle, she shifts the food box to the side. In another shot, when Ethel starts stabbing the boy, the box has moved adjacent to the body.
Whenever Ethel kills someone, by slicing to death or otherwise, blood is on them but no wounds are visible despite the cleaver making contact with skin.
After Ethel kills her grandmother, she tries to retrieve the keys by mutilating her hand. Despite her stabbing the hand continuously, there are no wounds on it. Also, if she had been stabbing it for so long it could have easily been sliced off and Ethel could have gotten the keys.
When Ethel is repeatedly slicing John to death at 44 minutes in, it's obvious that she's beating the weapon onto the victim's back, so no blood is visible on the shirt--and there are no accompanying slicing sounds either.
When the delivery boy gets smacked in the head with a glass bottle, no blood appears on his head.
As Ethel stabs the delivery boy with a piece of the broken bottle, it's obvious that she's slapping it on the wound area and that no gore sounds accompany it. Plus, there's clearly a large blood stain on his shirt, which would've taken some time to form.
The blood used in the movie is obviously red paint because of the film's low budget. An example is when Ethel is shown washing all the blood off her meat cleaver; it takes a while for the "thick paint" to evaporate from the knife, when authentic blood could easily wash off it.
Several shots that intercut frequently have the audio and visuals mismatched.