When Wendy escapes from her cover, the monster comes right behind her in another shot, but in the next shot of her there is clearly nobody behind her.
When Bruce is shown dead after his head was bashed by the monster, his face doesn't match the one shown in the shots before.
When Wendy recommends exploring the house and Tanya agrees,
but Bruce's hand isn't behind her head, but in the previous shot of her it was shown that he put it there.
When Bruce runs from the monster, he is pointing the flashlight at the monster, but when the monster is shown, no light is seen on it.
When Howard tries to break through the door, the wall also shakes, revealing that it's just a set piece and not a real wall.
Obvious fake head of Joel when it's found by Wendy and John.
When Bruce is running from the monster with Wendy, he pushes her in the room and keeps walking away which makes no sense, since there is no reason for the monster to follow him and not enter the room and attack Wendy.
In previous shots of the Unameable, it's hands sported long, claw-like fingernails. At the 55:26 mark, it holds up it's right hand in silhouette, and it's obvious it is a normal hand, without any prosthesis, makeup or effects.