Revealing mistake: When Max is talking to his therapist on Zoom, you can see on the screen that he hasn't "joined audio" for the Zoom call, meaning that he shouldn't hear her and she shouldn't hear him if it was, in fact, a real Zoom call. (00:20:24)
Revealing mistake: Nick is supposed to play the saxophone, and his cheeks are puffed full of air, but they do not move at all. Playing the saxophone requires blowing into the mouthpiece, which moves the player's cheeks, mouth, and lips. (00:57:00 - 01:06:55)
Revealing mistake: The "horse" in the stall on the left has to be fake. With the exception of a barely visible half-blink, it does not move the slightest when the girl (while handling the reins) walks around its head, and a couple of girls are touching (pretending to be removing) the saddle. (00:29:25)
Revealing mistake: When Jason is getting stabbed to death, there's a quick 2-second shot showing him being stabbed repeatedly in the stomach, with part of his intestines hanging out. But if you pay close attention, there's no actual damage occurring with any of the stabs, revealing it's just an effect. Likely either a collapsible prop knife or a CGI knife being stabbed into him.
Revealing mistake: When River and Eden are talking in front of the kitchen window, every other camera shift alternates between different backgrounds - still/blurred versus clear/actively snowing. The props are also different. (00:14:15)
Revealing mistake: In the scene with Dante in his pink robe and pigtails, the dead guy next to him, you can see his nostrils flaring in one scene. (01:06:00)