Continuity in film is important. Filmmakers want their audience to be able to suspend disbelief throughout watching in order to enjoy the story. Following along with the plot and making sense of it is essential to stop viewers from being pulled out of the experience on screen. However, sometimes a plot point just doesn't make sense or completely contradicts something from either previous installments of a franchise or earlier in the same movie.
Slasher movies are particularly egregious for this, especially long-running franchises. The number of sequels, lore, and frequent cast and crew changes makes it a lot more difficult to keep track of continuity, but sometimes the plot holes just can't be overlooked.
Jigsaw
In 2017, Jigsaw continued the Saw franchise despite it seemingly coming to an end with Saw 3D. In the film, bodies are turning up across the city that have died gruesomely in ways that echo the murders of John Kramer, the Jigsaw Killer, who died ten years previously. Filled with the twists and turns that are hallmark of the franchise, Jigsaw aims to obfuscate the identity of the true killer through misdirection and flashbacks.
The biggest plot hole comes when an order is given for the body of John Kramer to be exhumed. In doing so, the police find that the corpse of missing criminal Edgar Munsen is in the grave instead. The plot hole comes when the police don't notice that the grave had already been dug up to replace the body hours earlier. Surely the police would recognize when a grave has been disturbed?
Halloween
The original 1978 Halloween film spawned a huge franchise and set a blueprint for slasher movies that followed it. Six-year-old Michael Myers murders his babysitter on Halloween night, and is subsequently committed to a mental institution. Years later, Michael breaks out of the institution to go on a murderous rampage and begins stalking legendary final girl Laurie Strode (Jamie Lee Curtis). With an iconic look and relentless nature, Michael Myers is slasher royalty.
However, when Michael breaks out of the institution, he steals a car and drives away. How did Michael learn how to drive? As a dangerously disturbed inmate who has been locked up since childhood, it's unlikely that staff were taking him out for driving lessons, and yet he drives off with no problems whatsoever.
Freddy Versus Jason
In 2003, slasher movie fans finally got what they had wanted for years: a movie where dream demon Freddy Krueger went up against supernatural hulk Jason Voorhees. Krueger manipulates Voorhees to come back to life again and start stalking the youth of Springwood in order to reignite his own legend and return himself. The two supernatural bad guys quickly turn on each other, as Jason can't resist the urge to kill in full view of people, ruining Freddy's plan.
As the animosity between the two grows, Freddy stops Jason from attacking him by creating a waterfall between them, stopping Jason in his tracks and reducing him to a quivering mess. This is due to Jason's original death as a child in the lake. However, in the intervening years and sequels, Jason has been in the water by choice, hunting and killing off teens multiple times. His sudden fear of water makes absolutely no sense.
The Texas Chainsaw Massacre 3D
The 2013 movie The Texas Chainsaw Massacre 3D was a direct sequel to the 1974 film. The town sheriff arrives at the Sawyer family household and demands that Leatherface surrender. The townsfolk of Newt then come to burn down the farmhouse and kill the Sawyer family. A baby girl is found at the scene, and is taken and raised by Gavin Miller and his wife. Years later, Heather Miller discovers her true identity as a Sawyer and travels to collect her inheritance, but unknowingly walks right into the path of Leatherface.
The main issue with the film is its entire timeline. Judging by Heather's age, it appears to take place around 20 years after the events of 1974. However, the film actually takes place in 2012, as seen on the grave of Heather's biological grandmother. Heather should be approaching 40 at the time of the film.
High Tension
2003 French slasher film High Tension, known as Switchblade Romance in Europe, is often lauded as one of the best foreign horror films of the last few decades. Best friends Alex and Marie arrive at Alex's parent's house for a weekend of studying. Once there, a killer arrives at the isolated home and begins picking off the inhabitants of the house. A huge lumbering man, the killer pursues Alex and Marie, with Marie taking on the traditional role of the heroine trying to save her friend at every turn. However, at the end of the film, it is revealed that Marie is actually the killer, and has had a psychotic break and manifested another personality.
The plot hole in High Tension is actually a large one, but also one that is up for debate. If Marie is the killer, then she has frequently been in two places at once, often witnessing the murders from a distance. The argument for this is that Marie is simply dissociating from her murderous identity. She is unaware that she is the killer, and creating her own reality and explanation of her knowledge of the murders.