In season 5A, when all the characters are writing their initials on the library bookshelf during the senior scribe, look carefully and you'll see "METS!!" written on the shelf next to the one the characters are writing on. Dylan O'Brien snuck that in there.
As of season 6, Tyler Posey is the only actor to have appeared in every episode. Season 6 episode 2 marks Dylan O'Brien's first absence. Holland Roden, though credited for every episode, was absent from episodes in seasons 2 and 3.
The Sluagh which appears as a prominent antagonist in season 5 was the creation of an avid fan who entered her media graphic render of the monster through tumblr under #TWCreatureFeature which won a contest set up over facebook.
Like Buffy The Vampire Slayer both small Californian towns feature a center of mystical convergence drawing supernatural creatures to the town, Buffy's being the Hellmouth and Teen Wolf's being the Nemeton.
Holland Roden revealed in a 2017 Panel that in a scene from Season 6 episode 1 "Memory Lost" where Stiles kisses Lydia in the cheek was not in the script, it was improvised by Dylan O'Brien and herself.
Holland Roden revealed in the 2015 PaleyFest Panel that the Banshee screams in the show are fake. Roden explained that the screams in Season 2 were real, but for the rest of the seasons she stage-screams and the crew have a lot of her screams banked in the productions. Roden also revealed that most of the actors who portray werewolves have also stopped growling in their scenes, Tyler Posey and Shelley Hennig are method actors and still growl, but Tyler Hoechlin has just been hissing for awhile.
The scene between Tyler Hoechlin (Derek) and JR Bourne (Chris) in the episode "De-Void", when Chris blew out the lighter was improvised. Jeff Davis liked it so much that he told them to do it for every take. It was eventually put in to the show.
In addition to the character, plot, monster, action similarities and that both are based on original films helmed by the series forerunners both shows prominently feature Bianca Lawson as a recurring, powerful fighting character; Marin Morrell in Teen Wolf and Kendra Young in Buffy The Vampire Slayer.
Dylan Sprayberry originally auditioned for the role of Young Derek Hale; but the creators of the show thought he was too young for the role, the role eventually went to Ian Nelson. Sprayberry later got the role of Liam Dunbar.
Dylan O'Brien in an interview, stated that if there is a spin-off of the series that he would not be apart of it at all. While Tyler Posey stated he would love to be a main recurring character on the show.
Ian Bohen originally auditioned for the role of Dr. Alan Deaton before Seth Gilliam was cast for the role. Bohen later got the role of Peter Hale, Derek Hale's (Tyler Hoechlin) uncle and the first season's main antagonist.
Dylan O'Brien has announced he will only be appearing in one or two episodes of the final 10. He was originally not supposed to be in any because of his busy movie schedule, but found some time to film a couple of scenes.
Crystal Reed, Holland Roden, JR Bourne, Ian Bohen, Linden Ashby, Susan Walters, Matthew Del Negro, Tom Choi, Melissa Ponzio, Sinqua Walls and Victoria Moroles all guest starred in CSI: Crime Scene Investigation.
Dylan Sprayberry, Tamlyn Tomita and Ivonne Coll have all portrayed respective family members to main cast characters on Glee (2009-2015). Sprayberry portraying the younger Cooper Anderson (older brother to Darren Criss' Blaine Anderson), Tomita portraying Julia Chang (mother to Harry Shum Jr.'s Mike Chang) and Coll portraying Alma Lopez (grandmother to Naya Rivera's Santana Lopez).
Crystal Reed (Allison) did not come up with the scene in S05E01 where Scott writes down Allison's initials on the book shelf with all the other seniors' initials. This was a false rumor spread by fans of Crystal Reed via twitter. Jeff Davis confirmed in an interview that it was purely the idea of the writers to pay tribute to Allison.
Jeff Davis said in an interview that there was actually a side plot for season 3 where Jackson would find out that one of the Alpha Pack members was responsible for his biological parents death and would proceed to seek revenge. The plot was cut off due to Colton Haynes leaving the show at the end of season 2 to play the role of Roy Harper, on Arrow.
At the end of season two when Jackson is seen standing up his werewolf eyes are blue this is due to the fact that he killed most of the 2006 swim team as the kanima because he was officially a shape-shifter from the night Derek gave him the bite of an alpha.
Danny mysteriously disappears after the season three ending and is not mentioned or heard from through out season four and the current episodes of season five. At the beginning of the lacrosse season in season four coach explains that Jackson and Isaac are gone but doesn't explain Danny absence however Jeff Davis has stated in a interview that Danny graduated earlier.
In the season 5b episode titled "Maid of Gévaudan" Crystal Reed returns two seasons after her character Allison's death, as Marie-Jeanne Valet, a peasant girl who killed the Beast of Gévaudan. She only appeared in flashbacks from the 1700's.
Each season of the show has a specific theme: Season one: The Argents, hunters, and the Alpha. Season two: The Kanima. Season three part one: The Nemeton and the human sacrifices. Season three part two: Japanese demons battling a Nogitsune which possessed Stiles. Season four: the Berserkers, Kate Argent returning, the Dead Pool, and assassins. Season five: The dread doctors and finding out what they did Season six part one: The Wild Hunt and Stiles being erased. Season six part two: The hunters and final battle
With the Season 5b final episode and overall season finale Apotheosis the character Arden Cho and Crystal Reed make their final show appearence, marking the exit of the actresses responsible for main cast characters that served as head hero Tyler Posey's Scott McCall's love interests.
Each season is foreshadowed at the end of the previous season. The season 1 ending shows Jackson receiving the bite foreshadowing him as the kanima in season 2. The season 2 ending shows the pack sign on the burned hale house door foreshadowing the arrival of the alpha pack. The season 3 ending shows Kate's return from the dead foreshadowing her and the berserkers. Etc.
In Season 5 episode 11 "The Last Chimera" Dr. Gabriel Valack (Steven Brand) mentioned to his orderly about Lydia's (Holland Roden) Banshee ability that she can use her powers to shatter a human skull as well; later in the episode "Lie Ability" Valack eventually meets his fate as Lydia's banshee scream shatters his skull, killing him.