Continuity mistake: After the Halloween party, when Cady walks into Damian and Janis' house, Damian gets scared and tosses all of the popcorn out of his bowl. In the next shot of Damian his bowl is full again. (00:29:05)
Continuity mistake: When Cady is confronted by Aaron in her room, she puts her drink on a pile of CDs, the CD pile starts to fall in the background as there is a shot of him, a split second later the cup is on the table next to the CDs, and the CDs are piled differently. (00:58:40)
Continuity mistake: When Cady runs into her teacher on the first day of school and spills coffee all over her, look at Tina Fey's pants. It looks like she is wearing rubber pants when the coffee is spilling down her front. The color of the pants also change during this shot. (00:02:50)
Continuity mistake: When Regina becomes angry, and writes the horrible message in the burn book about herself to frame Cady and the others, the handwriting changes between shots. Look at "DO NOT TRUST HER" and "She is a fugly slut." - The handwriting changes noticeably, and "fugly slut" changes a lot - she writes it so the "g" and "y" have curled ends, yet it cuts to her sticking her photo in, and the ends to the tails on the "g" and "y" are straight. The line under "fugly slut," and the exclamation mark both change too. (01:00:15 - 01:00:45)
Continuity mistake: When Cady comes home from her first day her father sets down his book on the notepad on the desk as he sees her. In the next shot as she goes inside the book has moved back. (00:05:20)
Continuity mistake: When Cady is outside on the field with Janice and Damian supposedly going to the back building, there is a very large shadow in front of the football uprights in the background (behind Cady) which keeps disappearing / reappearing. (00:06:15)
Continuity mistake: When Cady is trying to get out of going to Madison, watch the level of her Dad's glass of water. From the first shot of the table to when he picks it up whilst saying "I think Cady's old enough..." the water level lowers. (00:52:50)
Continuity mistake: In the opening shot of Cady in calculus class, as her voice-over says "I was so happy to get to math class", the big textbook sitting on her desk alternates from closed to open. This happens again after Cady says "So Cute..." (00:12:10)
Continuity mistake: When the Mathletes are entering the spring fling, Cady's hands are in her jacket. When the shot changes, her hands are out of her jacket and can be seen. (01:26:20)
Answer: Regina enjoys manipulating people, and she views Cady as a relatively easy person to manipulate, finding her more useful/amusing to keep in their circle. It's a running theme of the film that the friendships between all of the Plastics is very tenuous, because of the catty, manipulative nature of the girls.
Purple_Girl
Also, if Regina tried to ban Cady, people might think she was threatened by Cady. Better to keep Cady around and attack her in subtle, manipulative ways.