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.
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?
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.
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.
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.
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.
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.
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: Season 04, Episode 14: 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.
Factual error: Season 2, episode 20: 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.