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 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.
When Dark Alessa attempts to absorb Heather (thus killing her), her eyes turn blue and her skin becomes normal. However, when it cuts to a pan shot of the two embracing one another as the carousel spins, Dark Alessa's face still remains goth-like before cutting back to the close-up shot of them both.
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.
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.
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.
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, Bassett claimed on his 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.
Vincent claims in Revelation that the Brethren wear gas masks and miner suits because they believe they will become "corrupted by the darkness" if they don't. In the first film, the mining suits and gas masks are shown to act as a deterrent to attacking monsters in the Otherworld, and the cult's priestess, as well as several other members of the cult, are shown leaving the church without any mining gear.
Despite the fact that Harry constantly warns Heather not to visit Silent Hill, the shopping mall sequence implies that Silent Hill's supernatural forces can still reach out to her which means it would be pointless to avoid the town.
Silent Hill: Revelation character Suki claims that she became entrapped in the Otherworld after getting lost in Silent Hill, suggesting that any person can become entrenched in the alternate reality. This directly conflicts with Dahlia's statement in the original film that only Alessa can "open and close the door" to Silent Hill. Given that there is no evidence Alessa knew Suki, there would be no reason at all for her to have brought her into her world.
The retcon involving Dahlia's and Christabella's sisterhood may be related to another continuity change, in which Sharon is stated to be an orphaned baby that was infused with Alessa's soul. The script sheds further light on this, depicting a flashback sequence in which Lisa Garland hands Alessa a baby kidnapped from its parents, into which Alessa puts the good side of her soul. In the first film, Sharon is no one's child, having been willed into existence by Alessa's powers, just as Dark Alessa was willed into existence.
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.
In the first film, the fire that destroyed the town was caused when the pit of burning coals that was used to burn Alessa overturned, burning through the floor of the church and into the coal mines beneath the town. However, Revelation states that Alessa created the fire with her rage after the burning.
It is shown in the original Silent Hill that there are three versions of Alessa: a badly burned adult woman, a child containing her good side (Sharon), and a child containing her dark side (Dark Alessa). Revelation makes no reference to Alessa's adult form at all, and strongly implies that Dark Alessa is the one controlling everything in the town, leading to the implication that Alessa's adult form may not exist in the second film's canon. The script furthers this theory, showing Alessa's physical body becoming Dark Alessa during the fire, as well as several scenes of Dark Alessa shifting into Alessa's burned form.
Dark Alessa states that Sharon was created so that Alessa could be free of Silent Hill, and could learn to feel love. However, this is contradicted by Alessa's blatant usage of Sharon as a pawn in the first film. Had Alessa created Sharon to keep her from the town, the first film's events would not have happened, as Alessa would never have called her to the town at all.
Vincent tells Heather that in order for someone to temporarily escape Silent Hill they have to go through a rigorous and painful trial. Given that Claudia also hired Douglas Cartland to stalk Heather, it can be assumed that he also comes from Silent Hill although there is no given evidence that he had to go through the same trial Vincent did unless he isn't from Silent Hill, and if this was the case then how was he hired by a cult trapped in Silent Hill? It could be argued that Vincent set him up but there is little given evidence that he and Douglas ever actually knew each, the only indication is when they see each other from across the school, even then Vincent never addresses Douglas after confessing his true motives to Heather, in fact Douglas is never actually mentioned again after his death, not even by Claudia.
Douglas and Vincent were supposedly sent to look after Heather by Claudia, however Harry ends up being kidnapped because Heather cannot be taken forcefully and must go by her own free will, so why were either of them sent to stalk Heather if she can't be forcefully taken to Silent Hill? The motivation behind their presence is never stated.
Harry and Heather have been running away from Silent Hill their whole lives yet when Vincent and Heather attempt to rescue Harry from the town it appears that it only took them a few hours or even a few days to get there by car.
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.