Stefan says Mason only has superhuman abilities during the full moon. However, at the carnival Stefan learned Mason has superhuman strength even without a full moon when they arm wrestled and Stefan witnessed Mason's superhuman strength and agility when Mason fought with Carter.
When Caroline is sitting down in the dungeon on the Lockwood Estate and has blood on her mouth, the blood changes its consistency.
Damon was able to compel Tyler in Haunted before he triggered his werewolf curse, so why couldn't Katherine simply compel Tyler to kill someone to trigger his werewolf curse instead of compelling Jimmy?
Throughout the fight scene between Katherine and Stefan and Damon the blood stains on the back of Elena's top keep changing size and shape.
When Damon and Lucy meet, neither of them seem to know each other. However, it was revealed in History Repeating, that Damon had made a deal with Emily Bennett to protect her entire bloodline in exchange for sealing Katherine in the tomb, in order to protect Emily's bloodline, Damon must have been aware of Lucy's existence.
The blood stain on the back of Elena's top is considerably smaller in this episode than it was in the last episode.
When Bonnie is preparing to send Elena a message she tears a page out of her notebook, closes the note book and starts to write the message, then the camera angle changes so you can see what Bonnie is writing and her notebook is open and she is writing on a page that hasn't been torn out of it.
Elijah was unable to recognize that Elena's vervain necklace belonged to his sister, and his mother even though it was been in their family for one-thousand years.
Elena came to visit Katherine in the tomb with a full bottle of blood. She poured a little and half the bottle is gone.
Rose-Marie tells the Salvatores that Elijah is merely a foot soldier suggesting that he's one of many Original Vampires and lower in status. In Klaus however it is revealed that they're all part of a single small family. This could be explained in-universe as a lack of knowledge on Rose's part.
Stefan tells Elena that Klaus is the oldest vampire in the world. However, later it is revealed that the oldest vampire in history is actually Klaus' step-father, Mikael.
This could also be explained as a lack of knowledge. It is possible that since Stefan only met Klaus and Rebekah, he was unaware of the existence of the other Mikaelson siblings, especially Mikael, Finn and Elijah.
When Elijah ripped out a vampires spinal cord, it was not able to regenerate. However, InThere Goes the Neighborhood, Damon's eyes were able to regenerate after Pearl gouged them out, showing that vital organs are regenerative.
In the scene where Elena wants to get out, the key ring is on her thumb, few second later it changes to her index finger.
Katherine says, "Your heart still beats, struggling to pump whatever blood remains. When it's gone, your veins rub together like sand-paper. It's excruciating." However, in A Few Good Men, Elena says, "Maybe this heartbreak will be good for him. It'll remind him that he has one, even if it doesn't beat." This creates confusion to wether a vampires heart actually beats.
Katherine is seen wearing Elena's necklace to impersonate her. The necklace contains vervain, and as shown when Caroline turned and was wearing her necklace, it should have burned her.
Katherine mentions that she knows all about the Originals since she's been running from one for centuries. However, in Know Thy Enemy, she believed that the dagger would work on Klaus, meaning that she did not know that he was a hybrid and immune to the dagger. However, she is apparently aware of his hybrid status by As I Lay Dying, when she gets bitten, so he may have told her off-screen between that time.
When Vicki shows up to Jeremy as a ghost, she was wearing a different outfit than the one she wore before she died, the Halloween vampire costume.
When Caroline, Bonnie and Jeremy are at the Gone with the Wind screening, they are watching the part where Scarlett walks among the wounded Confederate soldiers at the Atlanta train depot. Later in the episode, when Damon has escaped and is hallucinating about Katherine, the scene playing from Gone with the Wind is Scarlett leaving the hospital as the Atlanta residents prepare for the siege, much earlier in the actual film.