Continuity mistake: During the Doctor and Sarah Jane's conversation in the café, when Sarah says that she missed him and he responds that she was getting on with her life, in a shot facing Sarah, the Doctor's hands are not visible repairing K9. In the next shot, facing him, his left hand is suddenly on the robot, in a position where it should have been visible in the preceding shot. (00:17:45)
Continuity mistake: When Mickey pulls the blanket off K9 and turns him on to ask if he has anything to get them inside the locked school, K9's right side panel is missing, exposing the wires and circuitry inside him. When he turns up in the cafeteria to help the others escape the Krillitanes, the missing panel has suddenly appeared. (00:34:10)
Continuity mistake: When Sarah Jane breaks into the school, she has her purse with her. However, after she runs out of the hall with the headmaster's office after hearing some unsettling noises, it disappears, as she no longer has it when she finds the TARDIS hidden in a storage room off the gym.
Continuity mistake: During the Doctor and Sarah Jane's conversation in the café while he's repairing K9, when Sarah mentions the events of "The Christmas Invasion" and the Doctor confirms that he was there, the Doctor's hold on a piece of wire from K9's insides changes between shots. (00:17:20)
Continuity mistake: When Cassandra, after possessing Rose, is checking out her new body in a mirror, when she starts jiggling up and down, her right hand changes position between shots of her and shots of her reflection. (00:13:40)
Continuity mistake: When Cassandra, possessing Rose, answers her phone, as Chip is telling her that Rose's accent is "Old Earth Cockney", she puts the phone to her ear twice. (00:14:55)
Continuity mistake: When Cassandra reveals her presence to the Doctor, she starts off by turning him to face her, then pulling his tie out of his jacket with her right hand and tightening the knot with her left hand, in a shot facing him. In the next shot, facing her, she has her right hand holding the knot of the tie. Finally, just before Cassandra reveals her identity to the Doctor, shortly later, which hand is holding the tie changes again. (00:23:20)
Continuity mistake: When Cassandra, possessing Rose, confronts the Doctor after he wakes up in one of the clone pods and tells him how she intends to kill him, her hand changes position between the shot from inside the pod, when she rests her hand on the front, and the next shot from outside the pod. (00:24:20)
Continuity mistake: When the Doctor is attaching the ring device to the elevator cable, the position of his hand as he's holding it in place changes. (00:34:10)
The Christmas Invasion - S2-E2
Continuity mistake: When the TARDIS crash-lands at the beginning, it comes to a halt in a corner of the Powell Estate's courtyard, by the garbage containers. Later, when Rose is watching through the window as blood-controlled people walk through the courtyard, the TARDIS has moved further out into the plaza. (00:23:05)
The Christmas Invasion - S2-E2
Continuity mistake: At the beginning of Rose and Jackie's conversation in the kitchen about the Doctor, Rose takes a piece of food out of the fridge. Moments later, when she rests her hand on the light switch by the door, it has vanished. At no point in the intervening time did she come anywhere near a spot where she could have put it down, and she didn't eat it either. (00:04:10)
The Christmas Invasion - S2-E2
Continuity mistake: When Mickey is explaining what pilotfish are, after looking it up on his computer, in most of the shots he's gesturing with his left hand, but there's one shot where he's using his right hand. (00:14:10)
The Christmas Invasion - S2-E2
Continuity mistake: When Daniel Llewellyn meets Harriet Jones, she pulls out her ID card when she introduces herself. The scene cuts to a shot of Llewellyn saying that he knows who she is, and when it cuts back to Harriet, the ID card has vanished without her making any movement to put it away. (00:16:10)
The Christmas Invasion - S2-E2
Continuity mistake: When Rose goes to look at the Doctor in bed while Harriet is speaking on TV, the bedding has changed from when it was previously seen. (00:28:05)
The Christmas Invasion - S2-E2
Continuity mistake: After the Sycorax ship has arrived above London, when Rose has decided that they need to shelter in the TARDIS and is explaining this while preparing to put a housecoat/dressing gown/bathrobe on the unconscious Doctor, the garment jumps between the bed and her hands in a few shots. (00:30:25)
Continuity mistake: When Rose gets into one of the hospital's elevators early in the episode, stock footage of an elevator descending is shown. When the Doctor, Rose and Cassandra (possessing her) go into an elevator shaft later on, it looks and is shaped completely differently. (00:34:05)
Continuity mistake: When the Sphere Chamber is first shown, there is a rolling staircase set up just below the Sphere. Rajesh Singh is shown climbing the staircase and nearly touching the Sphere. However, when the room is next seen, when Raj hears that the Doctor is coming and walks up to look at the Sphere again, the staircase has vanished. Later, when the Doctor and Jackie are shown into the room while Yvonne gives them the Torchwood tour, the staircase is still missing, and it's more obvious that it is no longer in the room. It does not reappear for the remainder of the two-parter.
Rise of the Cybermen (1) - S2-E8
Continuity mistake: In this episode's TARDIS scenes, the yellow padded seat is positioned so that it's on the side of the console closer to the door, so people sitting on it are facing away from the door. In all other episodes featuring this version of the console room, the seat is positioned on the side of the console opposite the door.
The Christmas Invasion - S2-E2
Continuity mistake: When the robotic Christmas tree attacks Rose, Jackie and Mickey in the Tylers' flat, it barrels through the internal window and destroys a supporting pillar separating the living room from the main hallway. Later in the episode, the pillar is seen intact. (00:13:30)
Continuity mistake: In the sphere room the Doctor briefs Yvonne and Rajesh on the void. As he sits down in front of them you see Jackie's feet start walking towards them but the next cut behind Yvonne and Rajesh facing the Doctor shows Jackie isn't there. (00:25:15)
Chosen answer: 1) When Stephen Moffat took over he ignored a lot of what had been developed before (there is not in-universe answer). 2) It would have killed Rose, so the Doctor absorbed the energy. His body regenerated before the energy could do a significant amount of damage that would prevent regeneration.