Other mistake: When Tiff picks up Chucky to throw him into the crib, you can tell that the Chucky doll is inactive and not on or alive. (00:21:20)
Other mistake: In the beginning, when Tiffany has just had her soul put into the bride doll, she is reading the "Voodoo for Dummies" book and Chucky tells her that the page she is looking for is in Chapter Six, page 217. When she flips to it, the camera shows the top of the page before panning downwards. On the top of the page it very clearly says "Chapter Eleven". (00:31:45)
Other mistake: At the end, when Jade opens Chucky's coffin, there are several large rats inside, despite being sealed and buried under 6 feet of dirt for over ten years.
Answer: As specified in Seed of Chucky it's a voodoo pregnancy. That's why the pregnancy last shorter than a normal one.
Mortug