Plot hole: When Zach Young's grandfather died, Zach was said to be the sole heir of the fortune. However, Noah Taylor (the grandfather) had a daughter that appeared several times while Mike Delfino was looking for Deirdre. Where was Kendra Taylor while Zachary Young was spending Noah's fortune on dresses for Gabrielle Solis? Why didn't she claim her share? She simply disappeared.
Look Into Their Eyes and You See What They Know - S5-E19
Other mistake: In an earlier episode where Edie and Susan are driving Martha Huber's ashes to the lake to be spread, Edie gets a flat tire and Susan changes it. In this episode, all the girls are driving with Edie's ashes and they get a flat tire. Susan asks "Does anyone know how to change a flat tire?" Did she forget how?
Other mistake: Throughout the sixth season Eddie Orlofsky attends school with Porter Scavo, and Danny Bolen, witch means they must be close in age. However, in season 6 episode 20, it is revealed that Eddie was in high school the same time as Danielle. This does not make sense due to the fact that while Danielle was in high school, Porter and Preston were about 10.
Visible crew/equipment: When Susan sits on the couch in Edie's house, just after she throws the bra behind her and knocks the candles over, a crew member in blue jeans can be seen moving on the floor behind the couch to the right.
Other mistake: In season 3 it's revealed that Mike Delfino's doctor at the hospital is Ida Greenberg's nephew. When Ida dies in season 4 her only distant relatives come from another state, and there's no trace of her nephew from Fairview.
Continuity mistake: In season 1 it's revealed that Lynette and Tom first met when working for the same company, and Tom left his girlfriend for Lynette. In season 7 it's stated that they were already dating in college, and Tom betrayed Lynette with her friend Renee.
Continuity mistake: During the scene where Gaby and John are in her bedroom talking, she takes a drag from her cigarette while John asks why she isn't happy, but never exhales the smoke from that drag. 23 seconds pass from that drag to the next one and she's in frame the entire time (it cuts between shots of the two of them together and Gaby's close-ups, but doesn't cut away from her). It's also worth noting that Gaby is never seen smoking again at any other point in the series and no explanation is given as to why.
Plot hole: It says on Mary-Alice's tomb stone that she died on September 26th 2004, which was a Sunday, yet in the first episode, she says that she died on a Thursday.
Audio problem: As Carlos is giving his speech at the barbecue, Julie sees Zach and says to Susan "Oh God". After cutting to a shot of Zach and his father it shows Julie and Susan again. If you look to the right of the shot you will see Carlos' face is just looking in front of him and not speaking, but you can still hear him giving his speech whilst he is in shot.
Love Is in the Air (a.k.a. What I Did for Love) - S1-E14
Continuity mistake: When Bree Van De Kamp is sitting on the couch talking to her marriage counselor, as she is doing so she picks up her purse and puts it on her lap, crossing her hands over it. When the camera switches back to her a second later the purse is back on the couch beside her in the exact position it was in before she picked it up.
Plot hole: Rex is in the hospital for having a minor heart attack and is awaiting surgery to receive a pacemaker. 1) He isn't connected to an ECG or any other heart monitor so no one is aware of his heart attack. 2) They ran blood tests and checked his potassium levels. They would have also checked the concentration of his medication in his blood. (This should have been done earlier in the season too when they first discovered his medications were not working.) Zero concentration of medication in his blood could throw suspicions on his wife just as easily intentionally giving him potassium.
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.
Continuity mistake: Caleb hits Melanie with a piece of lumber and she falls to the ground. She falls with her feet towards Caleb, but in the close up (and each scene after) her head is now towards Caleb.
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.
Visible crew/equipment: Right before Bree drives herself to the emergency room and is talking to Mrs. McCluskey, a crew member's face is reflected in the car's left wing mirror.
It Wasn't Meant to Happen - S2-E20
Factual error: Gabrielle visits Dale, her adopted child's natural father, at his high school and interrupts a pep rally for the football team's upcoming game. Dale, the team's Quarterback, even says that in the last game, he threw for 260 yards. The problem is that it is spring. Dale has just returned from a week in Florida for Spring Break. Football season is over. Basketball season should be over or ending soon. There couldn't have been a pep rally if the season is over.
Continuity mistake: When Lynette is feeding her baby, we see from behind there is a backpack on her chair then when the shot changes it is not there. (00:29:55)
Continuity mistake: When Lynette comes out of the pool, after getting her boys, she is wearing high heels, but when the shot changes to a distance, she isn't wearing anything on her feet. (00:13:15)
It Wasn't Meant to Happen - S2-E20
Continuity mistake: While Susan and Edie are at the Hooters style restaurant, the amount of beer, and amount of head, in Edie's glass changes from shot to shot.
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.