Continuity mistake: When Nica starts pounding on the window to try and get Madeline's attention, she goes from hitting it with one hand to both between cuts when the camera cuts outside.
Visible crew/equipment: When Chucky flips off Nica from outside, if you look closely, for about one second, you can see the curly, auburn hair of a crew member on the right side of the screen ducking behind the actress.
Revealing mistake: When Claire is bitten by Chucky and she pulls her sleeve up, you can see where the prosthetic is blended into her arm in the first wide-shot. In both that shot, and another shot, you can also see the prosthetic unnaturally "bulging" slightly. It's also very visible in the scene where she's strapped down and Dr. Foley injects her.
Continuity mistake: When Madeline goes outside with Chucky, as she walks through the door, she's holding Chucky tighter to her body between cuts.
Continuity mistake: After the doctor burns Chucky's hand, when Malcolm leaves the room, there's a tuft of hair on Nica's forehead. When the camera cuts, it's suddenly brushed aside.
Continuity mistake: When Carlos is getting Nica water in the morning (after she finds the message from Chucky), her mouth goes from open to closed, then back to open between cuts.
Visible crew/equipment: When Chucky finds Nica with her wrist slit, you can see a light reflected in Chucky's eyes, even though the room is dark. Obviously, a set light.
Continuity mistake: When Chucky is talking to Angela, he's standing straight and turns his head towards her. But after he asks "Where's Nica?", it cuts to a wide shot and his entire body is turned, not just his head. His body rotates back and forth like this, depending on the angle.
Continuity mistake: When Chucky pulls a scalpel out of the drawer, the scalpel goes from sitting at a slightly angle to totally flat between cuts.
Continuity mistake: When Nica meets Tiffany, when Tiffany makes the comment about expecting "somebody taller," her hands change position in her lap. Additionally, smoke appears over the ashtray instantly between the same two shots.
Continuity mistake: When Grace is arguing with Nica and asks if she's going "chop off [my] head?!", the way she's holding Chucky changes slightly between cuts. Her hand moves down his chest slightly.
Continuity mistake: When Madeline is trying to feed Chucky at the cafeteria, when she says "I can't believe he's talking already!", she goes from facing sideways to facing forward instantly between cuts.
Continuity mistake: When Grace gets annoyed at Malcolm and says, "You always bring that up!" when Malcolm asks about her arson, her hands go from apart to together between cuts.
Continuity mistake: When Nica and Malcolm are having sex, Nica's hand is wrapped much further around his neck/shoulders in the second shot.
Continuity mistake: When Carlos is unpacking Nica's bag, he takes out a small white box and her hairbrush twice between cuts.
Visible crew/equipment: When the camera cuts to Andy's close-up as he's punching in his safe code, you can see a set light reflected in his eye. There's nothing in the scene in front of him that could be creating such a reflection.
Continuity mistake: When Andy is about to set down a beer in the opening scene, you can see he's casting two shadows: one behind him and one on the wall in front of him. When the camera cuts, the shadow he's casting on the wall in front of him vanishes. (Likely due to the lighting being different for the setup.)
Visible crew/equipment: When Andy leans against his gun cabinet in the opening scene, you can see a bounce-card reflected in the glass.
Continuity mistake: In the flashback to the post-credits scene to "Curse of Chucky," you hear Chucky scream "Wait!" This never happened in the original scene.
Continuity mistake: When Carlos is killed by the three Chuckies with different weapons simultaneously, the editing gives away that three different prosthetic chest-pieces were used to accomplish the effect - one for each weapon. The blood and wounds on his chest and stomach are inconsistent from shot to shot as we see the different weapons slashing and stabbing him. His body also wriggles about a bit differently from shot to shot.
Answer: It's been years since the injury. Maybe she transferred her soul to another doll? Maybe she repaired her head? Maybe she copied her soul into another pristine doll as she can now inhabit multiple hosts at once? Take your pick, as it's a relatively insignificant matter.
TedStixon