Art split apart Allie's right hand down below the wrist, but her hand and wrist are clearly intact when she is attempting to crawl to her phone.
Art clearly removes sections of skin off of Allie's back but when she is crawling on the floor, there's only blood smeared across her back with no skin missing.
The morning after the fire, Sienna's hair is nicely straight. When she's at school it's wavy like natural texture. Then at the shop later in the same day it's back to straight.
When Art kills Barbara, he shoots her head off her body. Shortly afterwards, he sets Barbara off at the table and it's clear that her head is still attached to her body (e.g. when Art stuffs food into her open head).
When Art attacks Brooke, he breaks her knee; but later, when he knees on her, both knees can be seen intact.
Art's favorite acid is fluoroantimonic acid, which he has in a clear glass bottle. It is a superacid capable of dissolving even glass.
The events of the first Terrifier movie are referred to as the Miles County Massacre. This is a fictitious county, but the cops who arrived at the crime scene wore NYPD insignias.
When Jonathan is reading about Art on a web page, a paragraph begins with 'Without further adieu".
(at 55:00) When Sienna is talking to her mother in the kitchen, she has long hair, a nose ring, no necklace, and a striped sweater. When she answers the door, her hair is significantly shorter, the nose ring is gone, she's now wearing a necklace, and is wearing a completely different sweater. However, it is not Sienna that answered the door. It's a different character at a different house. The poor way that the scene was edited makes it very easy to think that it's Sienna answering the door.
At about 1hr 17 mins, when Barbara drops the wine glass, the stem's missing when the glass hits the floor.
As the actor playing the Truant Officer starts to turn to leave after talking to Sienna's mother he looks at the camera.
When Sienna is texting Jonathan at the carnival, the conversation window clearly has a typo, with messages marked as "deilvered" instead of "delivered."
The morning after her dream, as Sienna is drinking her coffee, a little gets spit out and she does a quick check to see where it went.
During the news report on the Miles County Massacre, it is mentioned that the body of Art the Clown disappeared. However, no mention is made of the murder and mutilation of the coroner in the morgue. It's not clear what the police ever did about the latter murder and its connection to the disappearance of Art's body.
In the opening scenes of the movie leading all the way up to when Art leaves the laundromat, he is clearly shown multiple times to be missing 1 of his eyeballs. Despite this fact, and excluding Sienna's 'Clown Cafe' dream sequence, the next scene with Art while he's assembling his wooden club of knives and nails while the little pale girl watches the TV, his damaged and missing eyeball is somehow suddenly repaired and intact without any explanation as to how.