Keeping track of the magical lore and laws in a fictional universe — or three — is never bound to be easy, but one would expect that if anyone was going to maintain canonicity, it would be the writers. This, however, cannot be said for the writers of The Vampire Diaries, The Originals and Legacies, with numerous plot holes and breaking of magical lore over the years.
Whilst this destroys the experience to an extent, we’ll let them off, they’re only human. Nonetheless, we’re looking at the top 4 times writers of The Vampire Diaries Universe broke their own magical laws or made silly continuity errors.
Vampire-Witch Hybrids

At the beginning of The Vampire Diaries season three, one of the most concrete laws of The Vampire Diaries Universe was established: that a person cannot be both a vampire and a witch. Rebekah made this clear in her story about the Mikaelson Family to Elena, when she says that her family were once witches, but one cannot be a witch and a vampire.
This rule was pretty much set in stone for the majority of The Vampire Diaries, and even spawned a separate storyline in which Bonnie’s mother, Abby Bennett Wilson, was turned into a vampire and lost her abilities as a witch, something Bonnie had a hard time forgiving Damon for.
With the arrival of the Heretics in season six, however, it became clear that the writers had plans to change this law. The Heretics, as described by Lily, were “vampires with witch powers; an aberration of Nature”. The writers here were explicitly making use of one of Nature’s loopholes, that a witch can be born without powers, and is known as a Siphoner. As a result, they can be turned into vampires and do not lose their ability to channel magic.
Whilst this was one of the most annoying bloopers, it did make for good entertainment, but we’d have preferred it if they would’ve just followed the canonicity they had already established three seasons prior.
Older vampires are stronger

One of the rules that governs the vampric world in The Vampire Diaries Universe, is that vampires get stronger the older they are. It’s a classic tale often seen in other vampire series/movies, and it’s one that we thought the writers would get right. Unfortunately, this is The Vampire Diaries, and the writers are prone to forgetting laws they themselves have established.
This means that sometimes, fights between vampires do not seem likely or realistic. For example, when Anna returns to Mystic Falls to release her mother from the Tomb, she fights with Damon and he is somehow able to hold his own. This is despite Anna being over 500 and Damon only being around 170 years old. This obviously doesn’t make sense, and Anna should’ve easily been able to best him. What’s worse, a couple of episodes later, Damon tries his luck with Pearl — who is a similar age to Anna, around 500 — and she is easily able to best him and is keen to remind him that she is 400 years older than him.
Destroyed photographs that keep appearing

This is one of the more annoying plot holes, because it keeps happening. Granted, it’s an easy mistake to make, and most viewers probably didn’t notice it, but the keen eyed of us will have noticed it straight away.
When Stefan arrives in Mystic Falls again, he brings with him a portrait of Katherine that he acquired in 1864, the year she turned them into vampires. This photograph is later burned by Bonnie in season two of The Vampire Diaries, and there is unlikely to be a copy, and even if there was, Stefan is unlikely to have carried two of the same photographs around with him for 160 years. Despite being burned, this photograph is later seen in a glass cabinet in season five of The Originals, so it seems as though the writers forgot that it was burned or hoped that nobody would’ve noticed.
This error is later repeated in season five of The Originals. In season two of The Originals, Klaus takes a photograph of the Mikaelson’s and baby hope, but the photograph is later burned to keep Hope’s existence a secret from Dahlia and Esther. Nonetheless, this photograph appears once again in season five of The Originals, despite being burned three seasons prior. It’s definitely unlikely there was a copy as it would’ve defeated the whole point of burning it to keep Hope safe.
This seems to be something that other fans picked up on also.
When was Mystic Falls founded?

Mystic Falls is the haven for our characters, but the writers experience a lot of difficulty over the years defining exactly when the town was founded. Although this doesn’t seem like a big deal, it is when you look at exactly how the storyline is connected with Mystic Falls’ founding.
The first continuity error comes in season one of The Vampire Diaries, when it is said that the town was founded in the 1860’s, this is despite a plaque at Mystic Falls Cemetery saying that the town was established in 1792. In season six of the show, it is noted that the Armory received a package from a town named “Mystic Falls” in 1790, however this both clashes with the sign at the cemetery which says the town was founded in 1792, and again with the established canon that the town wasn’t founded until the 1860’s.
It seems as though the writers are either unaware of the timeline surrounding Mystic Falls, or simply changed the founding to suit the storyline at the time and then changed it back again.