Plot hole: It is highly unlikely that the murderer knew Phil was going to put his ear to the stall when he heard the babbling. It is even more unlikely that the murderer is going to get him on the first stab through the stall. (Which also requires a lot of strength). We also have to assume that he spent time hanging out in the bathroom knowing Phil would go there to begin with, and that other two men with weak bladders were doing the same simultaneously forcing the victim to go to the stall to begin with. (00:07:45)
Suggested correction: The killer is incoherently whispering in a strange way in the adjacent stall to lure Phil to press his ear up against it. After stabbing him through it, the killer inspects the knife inquisitively, as if checking to see if he actually got him. While it's still not a terribly plausible scene, the killer's demeanor suggests that he encountered Phil in the restroom by coincidence and improvised the kill, rather than anticipating all of Phil's actions as part of a perfectly executed plan.
The general logistics and planning of the murder are a separate issue - because no, the murder was planned. The entry just says that it's "highly unlikely", putting it mildly, that the killer could guess the exact position Phil would pick to listen to the noise. Just a few inches up or down, left or right, make a huge difference. The killer looks at the knife admiring the results, because if he had any doubts that he got his victim, he'd be trapped in a bathroom with a screaming, wounded, angry Phil and plenty people who could come and help.
To be more clear, the correction here is that Phil had heard strange talking/whispering rather than music, which makes it at least a little more plausible the killer would think he might put his head up against the wall at a certain spot. Unlikely for sure, but unlikely isn't a mistake, it's just what movies do. Phil's death was planned yes, though it stands to reason the plan was more "surveil and strike when vulnerable" and less "wait for him in this particular stall we know he'll be next to."
Plot hole: After Sid and her friend get out of the car, they run away from it. Yet somehow while they are talking, the killer climbs out of the car and ends up behind them without them seeing a thing.
