1.) When the boarding house is referred to as the plantation home that the Salvatores lived in 1864. Stefan took Elena to the ruins/bare-bones of that home in "Lost Girls." That plantation ain't around folks. The boarding house was built in the 50's or something. Different houses.
2.) Vampires have super-hearing ALL THE TIME. This one I do not get. The show has made it perfectly clear that vampires need to actually focus on something specific to super-hear. That's why we get the close-up of their ears. That's why new-vamp Caroline told Elena to be quiet in "Kill or Be Killed" and concentrated very specifically on listening for where the boys were. That's why Elena had to tell very, very new-vamp Jenna to concentrate and focus, direct her hearing to Stefan and Klaus to listen in on their conversation. If vampires had super-hearing all the time without focusing, they'd go insane from the non-stop cacophony of noise and conversations. In other words, unless Damon wanted to torture himself and thus focused on the noises specifically, he wouldn't hear every time Stefan and Elena have sex, and in future-fics, vice versa.
3.) Vampires have heartbeats on The Vampire Diaries. I wondered about that myself because Damon didn't realize that it wasn't Elena he was kissing in "Founder's Day" which should have been a dead giveaway if vampires didn't have heartbeats, so I figured they did. Obviously, this could have tied into the whole super-hearing thing, but considering what was going on, it just seemed like this would be an instance where Damon was super-focused on EVERYTHING about the girl he was kissing, LOL! Plus, why would Katherine even bother trying to impersonate Elena with the boys because all they'd have to do is listen for her heartbeat (or lack thereof). So, yeah, they had to have heartbeats, I figured.
Then in "By the Light of the Moon," Katherine specifically mentioned heartbeats. She described the vampire-dessication process to Stefan while in the tomb: "How long have you actually gone without blood? I know you get desiccated in theory, Stefan, but in reality, it's much worse. Your heart still beats, struggling to pump whatever blood remains. When it's gone, your veins rub together like sandpaper. It's excruciating." Yet, I've read countless fics that not only mention the lack of a heartbeat in the vamps, but also Damon knowing it's Katherine as opposed to Elena due to the lack of heartbeat -- which he obviously didn't on the show because he thought he was kissing Elena.
4.) Vampire-siring. I get that most vampire mythologies make siring a big-deal, there's a relationship, a bond, etc. between vampires and the "children" they sire. That is so beyond obviously NOT the case in The Vampire Diaries. Damon has absolutely zero interest in Caroline, who he sired, and Caroline clearly has absolutely no bond to Damon whatsoever. In fact, the show explicitly made it clear that there is no sire-bond by having Damon point out the strange sire-bond between Klaus and Tyler ... which he figured must be because of the werewolf aspect. Implying that, yeah, there is no such bond with regular vampires. I've read many a recent fic that has Elena having some sort of bond with whomever her sire is (it's generally either Damon, Elijah -- what? really? -- or Stefan) and she has some sire-bond with them, and she is their "child." Seriously. How can does anyone watch this show and not get that this is NOT a thing in this show's vampire mythology by now? I do not get it.
ETA: Thought of two more ---
5.) Misspelled obvious character names. The recurring/one-off characters bug too, but I can mostly deal with that if the story is well-written otherwise, but when Stefan is spelt "Stephan" (HAPPENS SO MUCH!) or Caroline is spelt "Carolyn," I'm like, really? You can't look up the proper spelling? And, hello, we've actually seen BOTH of those two names written out on the show (Stefan's in 1.04, and then shown again in 1.05, so pretty dang early on!), so no excuse. And it really, really sucks when the writing is actually good overall because I just get thrown with the mispelled name every time!
6.) Elena loving Damon ALL ALONG. It's clear that the story is taking place canonically and then the writers throw in Elena realizing that she's loved Damon all along, you know even back when he was awful to Caroline and Stefan, and pretty much a jerk. Heck, at any point of season 01 when, yeah, she cared, but girl was SO NOT IN LOVE WITH HIM YET! But, nope, you see, she's secretly loved him from the moment she knew they met, but had just denied it. And that doesn't work for me because it's not canon.
So, what fanfic peeves do you all have? The last thread was so much fun!