Elena claims that only witches can't be compelled by vampires but werewolves and hunters of the five can't be compelled either.
Bonnie is able to get the cure out of the prison world despite the fact it took the hunters tattoo in order to undo the spell. However, it's believed that Bonnie channeled Qetsiyahs blood to get inside.
The way the fight scenes are presented often don't make sense power level wise.
In Season 1 episode 13 Children of the Damned Elena asks why Stefan's father wasn't in the Salvatore Tomb. He tells her that it wasn't built until after Giuseppe's death. So how was Lily supposedly buried in the Salvatore Tomb, as was described by Damon in Season 6?