When the two officers first enter Dr. Giggles house, Officer Reitz grabs a piece of cloth that got snagged on the wall. When he reads it with the flashlight, it reads: awood. It is in big, blurry black letters. Later on in the film when Officer Reitz is at the police station looking up files on the computer, he pulls out the cloth and the letters are different. They still read 'awood', but are now clearer and there is more writing on the cloth as well.
When Tamara is in the bedroom, she tosses ice cream on the mirror. She then starts to clean it off, but is actually smearing the ice cream leaving a huge stain on the mirror. When she notices the medical kit on the bed, she turns around. Behind her, the mirror is now clean and spotless.
When Dr. Giggles' house blows up, Jennifer and her boyfriend fall to the ground twice.
SPOILER: When Jennifer is wheeled into the operating room, the nurse finds blood on the floor and discovers two bodies. Later, after the doctor has killed Jennifer's surgeon, she runs out of the same door, but there's no blood.
When Dr. Giggles sticks the needle through the peephole and stabs Trotter with it, he retrieves the the needle back through the peephole, but not without squirting out some dark liquid. The dark liquid lands on the doorknob. In the next shot when Dr. Giggles opens the door, the doorknob is completely clean with no traces of the liquid.
In Dr. Giggle's waiting room full of dead victims, the actor without a shirt on is breathing. You can tell if you look at his stomach.
In Dr. Giggles waiting room full of his victims, the blonde corpse blinks her eyes.
When Dr. Giggles has a flashback of him as a kid with his father and they are looking at his dead mother on the bed, you can see her eyelashes blinking. She is also visibly breathing. You can tell by her chest moving.
When Officer Reitz is checking the records on Dr. Giggles, it states on the computer that the patient is 42 years old, but the officer says that the patient is 40 years old.
As a student of medical studies, even though he's insane, Dr. Giggles should know that hearts are only viable for transplant for about six hours. And that's under optimal storage conditions, not a pile of organs stored in a bucket.