Goofs

The toilet mirror reappears intact after being repeatedly smashed during the fight sequence. (Maybe someone poured 'the substance' on it. LOL)

When Elisabeth is first checking out Sue after transforming back after Sue's first 7 days, both women are lying naked in the lab. However, in one shot, Elisabeth's arm is in a white-toweling dressing gown as she strokes back Sue's hair, but in the next shot, she is naked again.

When Sue removes the picture of Elisabeth, as she shoves it through the hallway, the crack in the glass is gone.

(at around 1h 30 mins) Sue's boyfriend, Diego, looks down at the blood in front of his feet, and it is seen that he's wearing shorts. He then is shown fully nude, without the shorts he was wearing 7 seconds before.

The coffee table reappears after being shattered in the fight sequence.

She is provided with easily 5-10ml of activator she is only to use once but a 1ml syringe. Seems a waste of an important medication.

The hidden room Sue builds is impossible; no modern building has an unused, room-sized empty space in the wall cavity. Especially not in a contemporary high-rise apartment building, like Elizabeth's. In reality, the only way to construct such a room would be to sacrifice space in one or more existing rooms (which isn't what's depicted; Sue just hammers through the bathroom wall, to discover a large, dark, unused cavity in the wall-space).

A New Year's Eve special on network television would not feature top less Las Vegas show girls. There are children seen in the audience and the nudity would not be permitted on network television.

The film shows Elisabeth's pupil dividing into two pupils, which slide around her iris like the egg yolks in the beginning, but that doesn't make sense with what a pupil actually is: a hole in the iris around which the whole of the iris contracts and expands.

According to the plot, the substance divides existing DNA and duplicates the same person. But Elisabeth has heterochromatic green and hazel eyes, while Sue has homochromatic blue eyes.

With her last transformation Elisabeth turns out to be a deformed hunchback who looks at least 90 years old, but yet she can run long distances, run down and up stairs, and carry an unconscious body without a problem. It's possible that she is actually still only 50 but just looks much older, which is why she can do all these things.

Humans do not contain nearly as much spinal fluid as depicted, at least not all in one spot where it could be drawn out as quickly as Sue does. This is part of the science fiction of the mysterious Substance.

As Liz is so obsessed with conforming to the beauty standard, it makes no sense for her to go on stage as the monstrosity she becomes.

However, not only is her mind in turmoil at this point, but the film is an extreme satire, and not meant to be taken entirely literally.

At the cafe Elizabeth ordered a hot beverage, which appeared at the table without waitress delivering the order.

At about 18 mins, the casting call says "Auditions at the studio this Friday" but never mentions which studio, let alone its address or any contact info.

Sue calls Elizabeth ancient and puts down Elisabeth's version of the show in an interview. But many of the things Sue does on her iteration of the shoe are similar or identical to things Elisabeth did on her iteration. This is likely a comment on hypocrisy in the entertainment industry.

At the end of the movie Elizabeth's face slithers down the sidewalk and ends up on her "Hollywood Star". The orientation implies that the star would face the person walking down the sidewalk but when the sidewalk sweeper cleans the side walk it is shown left to right.

After Sue has pulled her four upper front teeth out, her upper lip shape doesn't change, it still holds the form of an upper lip with teeth behind it.

Just when Sue gets a bloody nose from not taking the matrix food the first time, there is a short shot of the sink, in the reflective chrome there is clearly a camera visible.

Several times a large billboard is shown just in front of the window of Elisabeth's apartment, even though judging from the city seen below, it's way too high for a typical billboard.

There's no way Liz could miss part of the preparation for the New Year's show and then just walk straight to the stage from the outside without meeting anyone who's been looking for her, or anyone else involved with the show for that matter.

Sue and Elisabeth seem to have two different minds; they seem to share no consciousness. After MonstroElisaSue is birthed, it and Sue are the same person with shared memories and self.

During the fight sequence, when Sue chases Elizabeth toward the bathroom, it can be seen that she is wearing trainers as she runs. However, Sue is meant to be barefoot for the entire scene.

Twice someone in a television studio is counting down to going on air, and they say each number. In real life, the person would merely mouth "two" and "one", not say them aloud.

After the duplicate's initial birth, there's no reason either Elisabeth or Sue have to be naked when transferring their consciousness to the other. While part of the movie's themes, it makes little sense in-universe.