Continuity mistake: Adam hands Katherine the note he found on the floor written by her aunt in a previous episode. Katherine is holding the paper up. When Adam walks around her in the next shot, the paper is gone and Katherine is only holding her keys/purse. The close up of Katherine (the very next shot) shows her holding the paper again.
Other mistake: In the scene where Lynette and Tom are fighting about Lynette having taken Kayla to see a therapist, and Kayla walks away, you can hear Tom yelling about Lynette having a distorted reality and that she can't pull crap like that. In the scene where Kayla comes back into the house after conning the twins into jumping off the roof, Tom and Lynette repeat the same dialogue again.
If There's Anything I Can't Stand - S4-E4
Continuity mistake: The amount of cream on Victor's chest during his rubdown changes between shots.
Now I Know, Don't Be Scared - S4-E6
Continuity mistake: When Carlos walks in to talk to Gabby about staying with Victor, she is drinking a glass of wine. She drains the glass except for a small sip, when she puts the glass down it's half full.
Mother Said - S4-E15
Continuity mistake: When Orson is on the phone with his lawyer, he sets down the notepad with a page half filled with writing at the front. However, later in the scene Edie walks into the room and the front page of the notepad is full of writing, plus there's an extra line in the first paragraph. No one was around to move the page as Orson was in the shower.
You Can't Judge a Book by Its Cover - S4-E7
Continuity mistake: When Lynette opens the trunk of her sister's car, the trunk can actually be seen opening twice.
Continuity mistake: At the end of the previous episode, an unknown someone can be seen circling Dylan's face on a picture of her and Katherine in the newspaper reporting on the Founder's Ball. In this episode, we see the cop who pulled Dylan over get back in his truck where a copy of the newspaper sits on the passenger seat. This time, both Dylan and Katherine are circled.
Now You Know - S4-E1
Continuity mistake: When Bree grabs the old lady's wrist to keep her from touching her belly, her thumb is on the palm side of the lady's wrist. Then when the shot changes, Bree's thumb is now on the back side of the lady's wrist.
Mother Said - S4-E15
Continuity mistake: When Tom tells Lynette that Kayla is frightened of her because she hit her, the bedroom door behind him closes slightly between shots.
Continuity mistake: After Gabby throws John out of the hotel room, she locks the door. But when Carlos leaves the room, he opens the door without having to unlock it.
Answer: Kayla was sent away in episode 16 (The Gun Song) in season 4, after falsely accusing Lynette of child abuse. She now lives with her grandparents - probably Nora's parents.
I'm curious why we never see her again, why she never talked about, and why Tom never mentions visiting her.