Continuity mistake: Tara is sitting at her desk and tells Max they will have sex the following night, and she teases him that she "Might even bring some props." Right as she says this line, the camera cuts. In the first shot, she has one pencil in each hand, but in the second shot, suddenly both pencils are in her left hand. The position of her hands changes between cuts as well.
Plot hole: Bonnie is the anchor for the other side and feels when supernatural beings die. How come she didn't know Katherine wasn't dead? Did she never think it was suspicious that she never felt Katherine's body after she died?
Suggested correction: Katherine died but didn't go to the other side. So she didn't need to go through Bonnie. She went to hell to Arcadius. Bonnie is not the anchor to hell, we don't know how going to hell works.
After everyone finally realises that Elena had been possessed by her Doppelgänger Predecessor, It is actually shown that Katherine were more or less dragged away from Bonnie before she could crossover to the Otherside. So we do have some idea as to how one ends up in Hell.
Suggested correction: Katherine was a human technically when she died.
Technically, she was supernatural because she was a doppelganger, so she should've gone through Bonnie.
And even she was a traveller.
Episode #1.4 - S1-E4
Revealing mistake: Mrs. Weston's baby is obviously a doll.