Other mistake: There's no way Arthur could have known Scott was 10 years old. In fact, it's highly likely he would not have known any of the boys he killed were 10 years old.
The Getaway - S4-E12
Other mistake: The tub Rita was found in was full of water, to the rim. Volume displacement dictates that when Arthur got out of the tub the water level would have dropped to well below the tub's rim.
Revealing mistake: When Valerie's dead body is on the table after being brought up from the ocen by the ice truck killer her nipples are hard. If she was dead her nipples would not have any reaction to the cold.
Continuity mistake: When Dexter is packing kill tools for the road he "packs light" and chooses a knife and hammer. However once he's at the hotel room setting up the kill room he unrolls his entire set of tools like he does at all the other murder room scenes.
Revealing mistake: When Dexter is fighting Liddy, in the van after kidnapping Dexter, the knife is a rubber fake knife. (00:38:15)
Continuity mistake: In the shot where Angel gives Dexter a gift from the recently deceased Maria's office, we see Masuka through the window in front of Dexter. Masuka then enters Dexter's office, showing Masuka in two places at once for at least two seconds. (00:30:55)
Suggested correction: That's not Masuka we see in the window. Not only is the person seen through the window wearing a different shirt, he's not bald like Masuka.
Revealing mistake: Sgt. Batista shows Dexter a dismembered leg with the hip bone protruding. The hip bone looks ridiculously fake, with plastic seams and the pin used to attach it to anatomy models visible. (00:26:05)
The Lion Sleeps Tonight - S3-E3
Visible crew/equipment: While at the cemetery a reflector (to brighten the actors' faces) is regularly visible in the DA's glasses.
Seeing Red - S1-E10
Continuity mistake: When Angel is at the bar, the girl he is talking to has a drink held up by her face when viewed from behind. In every shot from in front, the drink is on the table.
Sunshine and Frosty Swirl - S7-E2
Revealing mistake: After Isaak Sirko killed Tony Rush by sticking a screwdriver in his eye, Rush almost instantly drops dead on the floor. When Sirko's assistant, Jurg, wipes the screw driver for finger prints, you can still see a pulse at the base of Tony's throat. (00:54:05)