For the duration of the movie (after Jim removes his bandages from his arm), Jim clearly does not have a scar or wound from being inflicted with the knife used by his friend in self-defense.
The mask changes position after it's thrown out the window by Jim and lands on the ground.
After Brian's character beats up a puppet and Jim tells him to stop messing around with it, he tosses the puppet aside and James (Jim) can be heard laughing in his character.
After Jim takes off and places the mask on his friend, he gets possessed by Snix. In one POV shot of Jim, Brian's character has a splotch of red on his hand - revealing that the actor had touched the fake blood that was used for the slashed arm on Jim.
One scene of Brian's possessed character chasing after Jim, you can hear the level crossing and a train passing by - but the sounds are cut off as the shot intercuts with another.
When Jim fights off his possessed friend with a sword, he's clearly hitting the air or not touching the actor with the weapon. Also, when he throws the weapon and it breaks - the prop bumps the camera to the side, but the next shot is fixed.
When the mask appears behind Brian's character, Jim instructs him to look behind him to prove that Snix is there; the movie inexplicably jumps and somehow the Snix mask is in another part of the room.
After Jim retrieves the Snix mask and shows it to his friend to prove he's real, a voice offscreen exclaims "What happened down here?".
Jim goes under Snix's power by putting on the mask; Jim's friend (Brian Genk) runs away from him through a stairway, the shot cuts and Brian's character starts strolling along like nothing ever happened.