Continuity mistake: When Millie (in the Butcher's body) goes to see her friends for the first time, throughout the scene Josh has his phone in his right hand. When Millie appears and it cuts to Josh screaming, suddenly his phone briefly switches to his left hand, before switching back to his right hand in the very next shot. (There isn't enough time for him to have swapped hands between edits).
Continuity mistake: This is a little difficult to explain, so I'll do my best. When Char makes Millie/Butcher open the jail cell, in the first shot (a side-shot) it's clear that Millie is about to grab the third bar in from the edge of the door. When it cuts to the front shot, she's suddenly grabbing the second bar in. Then when it cuts back to the side-shot, you can tell she just let go of the third bar in.
Continuity mistake: When Josh's mom is about to cut the Butcher/Millie out of the chair, pay attention to her right hand, which is holding the knife. Right before the Butcher/Millie springs up, as she's talking to her son, her arm goes from down by her side with the knife pointed down, to up by her chest instantly between cuts.
Continuity mistake: When Josh is watching over the tied-up Butcher, he's sitting on the couch, eating snacks and watching TV. If you look, you can see he has a cut on his face to the side of his mouth. When he gets up to get a snack, the cut suddenly vanishes and is missing for the rest of the scene. The actor must have cut their face right before they shot the couch bit, and then the rest of the scene was filmed either after it healed, or before it happened.
Continuity mistake: When Char begins chasing the car with Millie, Nyla and Josh, they are on an empty road and there are no other cars around. Instantly between cuts, they are suddenly on a different road, in a different area, and there are a ton of cars around. Char started chasing them almost instantly, so there's not enough time for them to suddenly be on a different road in a different area.
Revealing mistake: When Millie jumps on her shop teacher's back, the teacher switches from the actor to a stunt double as they begin to tumble about. Noticeably, his hair suddenly gets much darker and has less gray in it. You can also get brief glimpses of his face, and it's obviously not the actor.
Continuity mistake: When the Butcher (in Millie's body) attacks her shop teacher, he throws her to the ground, and she drops the screwdriver literally right next to her head. It then disappears in the next few shots you see her. Then, when she gets up, it reappears in the wide shot, but is suddenly about three feet to the other side of where she dropped it. Sure, she's moving around a little... but she doesn't move around enough for it to suddenly be several feet on the other side of her.
Continuity mistake: When Millie (in the Butcher's body) chases Nyla and Josh into the kitchen, Nyla turns around and Josh is not near her. But then in the next two shots, suddenly Josh is right next to her on her left. He then disappears from Nyla's side when she grabs the mop, and reappears behind her in the next shot. The scene is cut very frantically, so it can be hard to notice at first, but it's very obvious if you look for it.
Continuity mistake: When Millie is hiding beneath the bleachers and the Butcher is standing above her, pay attention. In the low-angle shots of the butcher, the air is clear. But in the shots that are parallel with him, there is some light precipitation visible in the air. (It looks like very light rain or snow.) It goes back and forth several times as it cuts between the two angles. It can be kinda hard to notice it at first, but once you see it, it's obvious.