In the end fight, Pyramid Head gets hit with a sword that rips open the metal of his head, however in any scene after that, his head is undamaged.
The sound of the siren is altered between films, to make Revelation's siren sound more like that of the games.
When Heather is at the mall, it is daytime, as evident by the light from the windows. However, despite only being in the mall for a few minutes, when Heather exits, it is suddenly nighttime.
The nurses in the first film only react to light to come alive but in this film they react to sound instead.
In the first film, the Welcome Sign is on the right side of the road. In Revelation, it's on the left, although it's possible that it's a different entrance to the town.
Harry Mason pulls a pop tart out of the toaster literally a second after it popped out during the breakfast scene. There's no way it wouldn't have burned his fingers after grabbing it.
John Reed Hryszkiewicz is credited as playing the Labotomy Creature, which is spelled wrong as it's actually "Lobotomy" instead.
When Heather wakes up from her nightmare screaming and Harry gets inside her bedroom, the shot reveals the lack of ceiling on the set.
Shortly after the school corridor is transformed into the hell version after her introductory class, the camera pulls back from Heather. You can see track marks where the camera is obviously on wheels, leaving a mark in the dirt on the floor.
When Heather is giving her speech to the class, Adelaide Clemens' Australian accent slips and she pronounces the word "been" like the word "bean" (with emphasis on the letter "e").
After Heather shoots the Lobotomy creature in the head she throws his corpse to the side which is shown to still be breathing as evidenced by his rising chest.
Sean Bean's English accent accidentally slips when he wishes Heather a happy birthday and pronounces it as "Happy Bairthday" even though Harry Mason is an American character.
In Revelation, Claudia Wolf claims to be the sister of Christabella, the main antagonist of the first film. In the canon of Silent Hill, Christabella is the sister of Dahlia, and thus the aunt of Alessa. This makes Heather and Vincent cousins, something that would not have been intentionally added if the sister relationship from the first film was meant to be part of Revelation's canon.
When Heather is looking around an office in the asylum you hear an off tone sound (implying the telephone is off the hook), however the telephone is clearly in the position where this sound would not be heard.
The first film takes place in 2004 with the great fire occurring in November 1974. Sharon is 9 years old during the film's events meaning that she was created in 1995 while Alessa was born in 1965. However, M.J. Bassett has stated that this film takes place 6 years after the previous film (along with the trailer) which would make Sharon 15, not 17/18. This would mean that this film takes place in 2010 and judging by Sharon's age it appears to be 2012/2013. The official website for this film though implies that the first film's events were retconned from 2004 to 2006. It is likely however that Bassett and producer Samuel Hadida just assumed that the first film took place in the year it was released although this film also implies that Sharon was 11 during the first film's events supporting the implication that the time-line was retconned.
The public bus has the Canadian sign "Yield/Cédez" on the back of it, indicating that the film was shot in Canada.
When Heather enters the town of Silent Hill she stands in an intersection in the middle of town. Noticing the traffic lights, there is a big light at the top (Red), two small lights (Yellow/Green) and a large light at the bottom (Arrow?). These lights are specifically Canadian style traffic lights.
Heather receives her trademark vest on her 18th birthday and does not meet Douglas until a few minutes later but when his wallet is found on his corpse it contains a picture of Heather wearing the vest despite the fact that they only recently met that very day she got the vest.
The cult shown in Revelation, despite being stated to be the same cult as in the first film, has an entirely different set of beliefs that is almost completely opposite to that of the original cult. The first film shows that Alessa Gillespie was burned alive as a witch, due to the cult believing that she was "sin incarnate" due to her being born out of wedlock. Revelation, however, states that the cult burned Alessa so they could perform a ritual to impregnate her with a physical incarnation of their god, knowing that she would survive the fire. Initially, M.J. Bassett claimed on her blog that the Order (the cult seen in Revelation) and the Brethren (the name given to the first film's cult) were two separate cults. However, this is contradicted by Vincent's statement in Revelation that the Brethren are the most fanatical members of the Order. Similarly, the religious symbol of the cult is changed dramatically between the two films, to the point of being unrecognizable.
In the first film, Dark Alessa states that she brought Rose to the town because "this dream of life must end, and so too must the dreamers within it", and that it is "the end of days", making it clear that she wants to use Rose to kill the last remaining members of the cult. In Revelation, there is still a large group of cultists living in the Otherworld, who are even seen walking about town and leaving their mentally ill in the asylum. It is not possible that Alessa would not have known of their existence, and this makes the entirety of the first film meaningless.
The subplot involving Heather being wanted by the police as a suspect because her coat was found at Douglas's crime scene is a cliffhanger. It leads to nothing and is never resolved.
At the end of the original Silent Hill, Sharon and Dark Alessa recombine to become a complete incarnation of Alessa, containing all parts of Alessa's personality and powers. This complete version of Alessa is the child Rose takes home with her, thinking she is Sharon. In Revelation, Alessa's multiple incarnations are still separate, and Sharon is shown to still be only the good side of Alessa, with Dark Alessa existing in a separate body.