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.
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.
Audio problem: When Susan suspects someone might be in the house, the seduction music she has put on for Mike is drowned out by the score. When she accidentally hits Mike across the face, the score stops, but the music that was playing in the background has gone. The song in the background before now was music within the show, and it was loud enough to be heard around the house, so we should still be able to hear it.
Factual error: Lynette's neighbor tells her that the pot Lynette's sons gave her is really hers, the neighbor's, and she bought it in Costa Rica. She turns the pot over to show Lynette that the price is marked on the bottom, with a number followed by the letter p. One supposes that the p might stand for peso, except that the currency of Costa Rica is the colon, which consists of 100 centavos. No p in sight.