Revealing mistake: When Chucky's disembodied head tries to bite Andy in the beginning, watch closely. When the plaque he's mounted too leans forward suddenly as Chucky snaps at Andy, you can see a rod underneath of it, which was used to make it move and lean forward by the puppeteers.
Continuity mistake: When Andy is grappling with the "buzz-cut" Chucky and trying to rip his clothes off, at one point the camera cuts from a medium shot to a closeup as he rips Chucky's overalls down. In the medium shot, Chucky's left arm is reaching for Andy's wrist, but isn't holding it. But in the closeup, suddenly Chucky is grasping Andy's wrist.
Continuity mistake: Dr. Foley burns and melts the Chucky doll on his right hand with a lighter but when it cuts to a wide shot, Chucky's hand is completely fine.
Continuity mistake: When Nica-Chucky stomps Foley's head in, you can see a ton of brain matter coming out in the wide shots. But in the final closeup, there's only one or two little pieces.
Continuity mistake: When the security guard looks in the window of Tiffany's car, there's no snow behind him, even though it's falling in every other shot.
Continuity mistake: When Carlos dies, when his head falls over, the blood on his face changes between cuts.
Continuity mistake: Malcolm goes from watching Madeline's body being taken away to looking at Dr. Foley between cuts when he makes a quip about how Foley must be having a "bad day."
Continuity mistake: When Nurse Ashley takes the shoe from Madeline, Madeline's hands change positions repeatedly between cuts.
Continuity mistake: When Dr. Foley pours himself a drink while Nica is hypnotized, the bottle cap goes from right in front of him to about a foot to his left instantly between cuts. It then reappears on the bottle a few shots later despite him having not put it back on.
Revealing mistake: When there's a close-up of the needle going into Nica's arm, you can tell it's a fake needle - the tip is just flat, and there's no hole, unlike a real hypodermic needle. You can also spot where the fake needle "retracted" in the center.
Continuity mistake: When Foley turns to Nica with the needle, in the shots over his shoulder, he's holding it below shoulder level (to the point it's not visible on camera), while the shots facing him, he's holding it at shoulder level.
Continuity mistake: When Foley is getting the needle ready to hypnotize Nica, right after he squirts a little out, the amount of fluid in the needle is suddenly about 75% smaller instantly between cuts. (It's not because he just squirted some out; it's instant between cuts.)
Continuity mistake: When Nica is trying to tell Dr. Foley about how she thinks both dolls are alive, when he leans forward, the lighting on his face changes from the side shot to the front shot.
Visible crew/equipment: When Andy gets "Buzzcut Chucky" out and drives away, you can see a very clear reflection of the set lights in the side of his truck.
Continuity mistake: When Claire is decapitated, in the first few shots, it's clear her head is going to roll away towards the door. However, after a quick cut, we then see it roll in the opposite direction towards Chucky.
Deliberate mistake: When the cannister rockets through the ceiling window, the pieces of glass from the initial impact never fall down. Nor are they on top of the window in the next shot. They just sort-of vanish. Additionally, the cracks don't match from the first shot to the second time we see it. There's suddenly less cracks which is odd because it's a digital effect.
Continuity mistake: When Chucky slams the compressed air cannister down, he goes from having his eyes squinted to wide open instantly between cuts.
Continuity mistake: Claire goes from facing her head up to facing sideways when Nurse Ashely comes into the room after she's strapped down. In the very next shot, she's facing up again.
Continuity mistake: Carlos' hands go from on Claire's strap to off it between cuts after he clips the final strap while tying her down.
Continuity mistake: The first strap Carlos puts around Claire moves from above to below her beltline, and then back again between cuts as she's being strapped down.
Answer: Chucky states that he found a new voodoo spell in the film that allows him to control multiple hosts at once. It's almost like a copy/paste of his spirit. Andy still has the original Chucky, whereas the rest are duplicates.