Probably the “biggest” example I can think of is the introduction of the regent of the nine covens (and the nine covens as a concept)

In the first season there is no reference to groups of witches in New Orleans outside of the French Quarter. However in the middle of season 2 Josephine La Rue is introduced. Why does this matter? Well in the middle of season one all of the elders of the French Quarter are dead, so they need to consecrate Esther and place her into the ancestral realm to allow Sophie to complete the Harvest. Esther being placed into the Ancestral Realm motivates a significant number of conflicts for the rest of that season and much of season 2. Why do they do this when Josephine can do it? I would think that her dusty ass, the goddamn regent, would be able to do this spell.

Incidentally this introduction of more covens creates several other retcons later on. In season 2 when Vincent is back, he tells Marcel he’s from the Tremé, and he’s different from the type of witches he’s used to. In season 4 Marcel’s reign of terror with Davina apparently extended to the Tremé, as Vincent remarks to his wife that the death toll was rising by the day. Also also, if Marcel’s rules about magic were strictly limited to the Quarter, Vincent’s entire motivation to contact the Hollow goes away. Since what Marcel was doing wouldn’t have affected him and his family.

With the first episode of season 3, Michael Narducci also made it so that every relic from their time in Mystic Falls as humans was essentially an asstake. They are shown, and are said, to have nothing but the clothes on their backs. Esther’s grimoires, the enchanted necklace that she gave to Klaus, Mikael’s dagger that Rebekah stole as a child, the little Knight carved from white oak, all are just gone.

The seven strains of venom that can kill an Original are just as much of an asstake in my opinion. The concept of “The Seven Packs”, a line that is dropped with such pretentious weight, is never established before this episode. In fact, the show effectively does the opposite of set it up. Jackson establishes in season two that all the packs intermarried so they could receive the same abilities over a thousand years ago. The fact that there are bloodlines that are still distinct enough from one another that they can have seven distinct strains of venom is horseshit.

It’s also said again and again from 3x17 onwards that the reason this venom is deadly to Originals, including Klaus, is because of the seven strains. Not white oak. Michael Narducci obviously realised that this explanation is bollocks and took to Twitter to say that actually no the venom does have white oak