Continuity mistake: On 7/8/05, Mimi mentioned to Rex that she woke up in a cell that day, referring to her time in prison. But it was actually still just later in that same day that she was sentenced, before which she was out on bail. So she'd gone into prison that day, but she hadn't woken up there.
Plot hole: When Zack Brady was hit by a car and severely injured in January of 2006, Dr. Lexie Carver urged Bo and Hope to take him off of life support as Zack had no chance of recovery. To encourage them towards this decision, she pointed out that Zack's liver might be a match for Claire Kiriakis, a baby who was dying of liver failure. However, Lexie failed to mention that liver transplants can be done from live donors because the liver regenerates. Dr. Carver would certainly have known that.
Plot hole: On 7/16/04, Mimi heard on the radio that her brother Patrick was dead, and then she went to Alice's and told her mother Bonnie. But the news never releases the names of victims of any type of fatal event before the family has been notified, so Mimi never should have heard news like that on the radio.
Continuity mistake: In the episode airing 8-29-07, EJ and Sami are locked in the steam room. When EJ short-circuits the fan and falls off the ledge, he falls forward onto the floor and lands face-down, which you can see by the direction of his feet as he falls. Yet, in the next shot, one second later as Sami leaps to her feet and runs over to him (which, I can't imagine how she leaped up that fast, being as pregnant as she is), EJ is suddenly lying on the floor on his back.
Other mistake: On 9/26/03, there was a quick montage of all the suspects in Abe's shooting superimposed on a shot of the comatose Commander Carver. It had been well established that the seven suspects were Kate, Lucas, Sami, Tony, Rex, Victor, and Nicole. However, Lucas was missing from the montage.
Plot hole: The week of 2/23/04, Marlena tried to kill Shawn by stabbing him to death while he slept, but Jan was in the bed instead. By this time, Marlena had killed seven people, several of them by slashing. So she was a skilled and accomplished killer. Yet somehow, when she was done with Jan, Jan was in good enough health to get out of Shawn's apartment, get out of that building, walk to the Kiriakis mansion, and get upstairs to Nicole's bedroom.
Continuity mistake: On 9/7/04, John and Bo place a shiny metal paperweight on their map, and the name of the island, "MELASWEN," is reflected as "NEWSALEM," for "New Salem." But even though "MELASWEN" is indeed "NEWSALEM" backwards, the first word would not actually reflect legibly as the second. The letters E, L, S, and N should appear backwards.
Continuity mistake: At the end of the 8/3/05 episode, Sami jumped out of the cake, walked directly to Lucas, and said, "I am going to give you a birthday you'll never forget," and then she kissed him. At the beginning of the next episode, Lucas asked, "Sami, what the hell are you doing here?" when she first popped out, and there were about two more lines of dialogue between the two of them before Sami delivered the same line as the day before and kissed him.
Continuity mistake: On 7/13/05, Bonnie complains of a stiff neck from sleeping on the sofa bed in Mickey's den. However, when Mickey first suggested that he, Bonnie, and Maggie live together, he specifically stated that there were three bedrooms in the house and they would each have one.
Plot hole: On 12/28/04, Sami and Belle talk about how they used to play with Barbies as kids, and how Sami made Belle promise that she would let Sami get married before her, or else she'd tear the head off of Belle's Barbie doll. But when Belle was born, Sami was already a teenager; Sami actually kidnapped Belle and tried to sell her on the black market. So Sami and Belle were never kids together, and the described interaction is impossible.
Plot hole: On 8/9/04, Patrick told Hope that she would be able to jam the force field by "hitting the keys in an even and then an odd number sequence." This phrase is completely meaningless, but it is all the information that Patrick gave Hope about the keypad, which we had previously seen was a regular nine (or ten) digit keypad. And yet, Hope was able to get back through.
Factual error: The weeks of 4/26/04 and 5/3/04, Marlena willingly went under the influence of sodium pentathol so that she could remember the killings and confess to them. But sodium pentathol does not restore repressed memories. It can be used to cause someone to speak against their will, but only about things of which they are cognizant.
Revealing mistake: On 12/19/03, Sami was attacked by the masked killer in an alley. She breaks away from him or her, does a backflip, and lands near the opposite wall. Ali Sweeney, the actress who plays Sami, has straight blonde hair. During the flip, the stunt double also had blonde hair, but it was very curly.
Continuity mistake: On 11/26/03, there was dialogue from several characters indicating that the Thanksgiving celebration would take place later that day, i.e., that it was already Thanksgiving. However, when Thanksgiving actually started on 11/28/03, it was the morning of the following day.
Plot hole: On 1/13/04, Marlena provides what may be the deciding testimony in Tony's hearing. Marlena is introduced to the court as a "criminal psychiatrist," despite never before being credited with that specialty. Her brief statement had nothing to do with psychiatry or the facts of the case, and didn't even provide any insight of the sort that a character witness might. She basically said, "I don't think Tony is the murderer." I don't see how this would have any bearing on the case, or would even be admissible.
Revealing mistake: On 2/17/06, Kate was looking at a picture of Carrie and Austin on her cell phone. The cell phone wasn't held perfectly still, and whenever it moved there was a jerky movement of the picture. The picture had been added in post-production, rather than actually being displayed on the phone, and it became obvious every time the image moved.