Plot hole: The movie at best is being dishonest with the ending of the chase scene; to keep Dr. King as a possible suspect, the gunshot happens so shortly after she went out of frame (with no cuts) that (since she is -NOT - the killer) there is no possible way that the gun could have ended in her hand. There's also no possible way for the culprit to escape, to find Hassan in the crowd and be there at the perfect moment, etc. (01:16:35)
Plot hole: It seems unlikely any mental institution would let a patient out to visit home for any length of time as happens in the movie with the killer and his mother.
Suggested correction: Unlikely, but still possible. Almost every workplace has a situation in which a person is bending the rules.
Plot hole: The killer has been living in the (apparently big enough) crawlspace between walls and floors, for over a decade. There is no explanation (or rationale) given on how he moved the various objects inside every room, closed doors, fed when the house was not occupied, hid the various corpses and, silly detail, concealed his own BO and the stench of his own living quarters, which is something hinted at when they find out his hideout. It is explicitly said that he is agoraphobic and does not get out, ever.
Plot hole: For the murder on the highway to happen, the killer (who is not a mastermind, but someone who has a daily job and who never killed before) had to sabotage the car of the victim in such a clever way that he'd stop at a specific point and time (so, predicting also he'd escape out of town and do it through a certain route, which he had no way to predict), and that he'd react to the mechanical problem waving at passing cars walking in the middle (literally in the middle, not the side, or the emergency lane; perfectly in the middle of it) of a highway, where he can drive his semi into him - all while the fog is incredibly thick. Needless to say, it is all a bit too convenient.
Plot hole: Marion rips the paper into pieces and flushes them down the toilet. We see all the paper go into the toilet. Later when Lila lifts the toilet seat, there's one piece conveniently placed under the seat that couldn't have gotten there.
Plot hole: After the first murder in Pam's dorm room there should be blood everywhere but when Pam returns to change her dress there is only a single stripe of it on the bedsheets.
Plot hole: At Tim's funeral, Olivia's attorney tells her that Tim had himself removed from Olivia's Last Will and Testament, proving he loved her and didn't want her money. However, an attorney could not legally change a client's Will without that person's knowledge and drafting a revised version.
Suggested correction: Not true. Anyone can have themselves removed as a beneficiary of a will. This is done by signing an Affidavit of Disclaimer of Inheritance at such time as someone becomes aware that they have been included in a will as a beneficiary. This is what Tim meant when he said he had removed himself from Olivia's will.
Plot hole: Blore's death is fairly absurd, since the killer couldn't plan that he'd be standing, with all the possible room outside of the house, exactly in that spot at that distance from the window, with a ton of bricks that are precariously balanced on stone spheres that survived storms and heavy winds but somehow are loose enough to require a gentle push to fall down. (01:25:30)