Revealing mistake: When Beast-Leonard is killed by having the Seal ripped from his chest, when he disintegrates, the shot goes completely out of focus as he begins to "burn up." All the more puzzling because the effect is computer-generated and only the background is real footage. Perhaps the background shot went out of focus and they had to purposely blur the effect to match? Either way, extremely sloppy work.
Revealing mistake: When we see a vision of Child-Alyssa walking down the street with a sort-of vortext behind her as the cult-members flee, watch closely and the entire shot is comical. All of the extras are barely moving at a jogging-speed despite supposedly running for their lives from her. Several are visible running towards her direction or even directly behind her despite supposedly fleeing from her. Several extras veer off to the side before exiting frame even though there are buildings on either side so there's nowhere to go. And several others still are visible running left-to-right or right-to-left from either side of the frame, again despite there being buildings on either side. Obviously all of the extras were filmed in front of a green-screen being told "just run around a bit" and then added to the scene. The shot makes no sense if you pay attention.
Revealing mistake: When Rose is in the mirror, her hand is under the husbands, when she removes it his hand moves along with it, but his real hand doesn't move.
Revealing mistake: When the blind guy puts the Seal or key into his chest you can see for the first two times he "hits" it to his chest, that he is not hitting himself at all.
Revealing mistake: Throughout the final fight between Pyramid Head and the Missionary, you can tell that Pyramid Head's mask and sword are very light-weight and made of rubber or another soft-material. Ex. When Pyramid Head hits Missionary out of the arena with the hilt of his sword, in the wideshot, if you look through the flames, you can actually see the sword bobbing about in the air before the tip hits the ground as the actor can't keep it steady.
Revealing mistake: During the final fight, when the Missionary is "sawing" through Pyramid Head's metal helmet, if you look closely, the helmet has been pre-scored and that the actress playing the Missionary is just running the blade along the pre-cut line in a "sawing" motion. The "metal" of the helmet also blatantly bends like rubber during the moment, revealing it's not really metal.
Revealing mistake: When Heather is in Silent hill, (right after the Mannequin part.) and talks to Alessa's mother, if you look at her nostrils you can see two rings of clean skin. (the rings are there for make-up reasons) but then after the flashback they are gone.
Answer: I believe your theory is correct, it appears to go out of focus to help cover up some unconvincing CGI. The final explosion looks very cartoonish.
BaconIsMyBFF