A Weekend in the Country - S3-E3
Revealing mistake: When Orson and Andrew are sitting in the diner, the same man in the green shirt keeps passing the window.
Like It Was - S3-E4
Continuity mistake: When Gabby and Carlos are driving in the car, Gabby's hair keeps switching from behind her ear when the shot changes.
Other mistake: Lynette is talking to Susan about Zach's infatuation with Gaby. Susan asks how Zach can afford so many expensive gifts for Gaby, to which Lynette replies that he inherited his grandfather's money when he "turned 18, a month ago." However, in the previous episode, Zach reminds Gabrielle that he is two months older than John Rowland, who, it was mentioned, turned 18 in season 1, two years before.
Getting Married Today - S3-E23
Continuity mistake: Gabby's hair in the beginning has her bun high on her head and her tiara is visible from the front. When Bree walks in, and when she overhears Victor and his father talking, her tiara is farther back on her head and no longer visible from the front.
Continuity mistake: When Nora dies, Lynette stands up and strokes her hand over her chin, so a little blood stays on her chin. During the yelling scene of Lynette towards Carolyn, the blood on her chin changes constantly between shots.
Continuity mistake: Susan's house is burned down by Edie towards the end of season 2, but at the beginning of season 3 the house is exactly as is back to normal and as it was before the fire.
Like It Was - S3-E4
Continuity mistake: When Bree is talking to Vera about Andrew and her husband, Vera is holding a nearly empty glass of tea. She then fills the glass to the top, but when she picks up the glass after doing so, it's nearly empty again.
Like It Was - S3-E4
Continuity mistake: When Bree goes round to visit Vera, the camera is reflected in the door as Vera opens it.
Sweetheart, I Have to Confess - S3-E6
Visible crew/equipment: When Susan starts talking to Ian after trying to hide from him, a boom mic and pole are reflected in the rear window of her car for a few seconds as the camera pans over it.
Other mistake: When Lynette and Tom are waiting for their kids at the bus stop, the bus that drops off their boys says "Briarcliff Academy." The boys actually go to Barcliff Academy.
Continuity mistake: Carlos swiped every thing from the table but a fork appears behind Edie. (00:18:35)
What Would We Do Without You? - S3-E22
Revealing mistake: When Lynette tears open Tom's shirt before making love to him, you can see there are already scratches on his chest before she scratches him.
Continuity mistake: In the scene where Carolyn shoots Lynette, Lynette is wearing high heels when she is originally shot. But when Lynette falls into Art's arms, she is wearing shoes without heels.
Continuity mistake: When Bree throws a glass of water of Orson to wake him up, the upper part of his shirt goes from being spotted with water to being soaked.
It Takes Two - S3-E2
Continuity mistake: While preparing for Bree's wedding, Susan is seen wearing full hair curlers for the entire day, yet on the day of the wedding, her hair is straight without a single curl.
Sweetheart, I Have to Confess - S3-E6
Audio problem: Right after Nora kisses Tom when she says "hey, lets have some more wine.", her lips don't match the words.
Continuity mistake: During the dinner scene, Ian starts to pour his mother some wine, but in the very next shot, he starts to pour it again.
Continuity mistake: When Lynette sends Kayla upstairs, her bag is seen both on her shoulder and in her hand between shots.
Continuity mistake: When Gabrielle is at Bill's home drinking a glass of wine, the amount of wine in her glass changes several times as the camera changes position, without Gabrielle drinking any of it.
Sweetheart, I Have to Confess - S3-E6
Continuity mistake: During the scene with Tom and Nora in the pizza parlour, a wine bottle and a glass are knocked over in an argument. When they pan back to the table as Nora is leaving, the full bottle and the glass are back on the table.
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.