For the number puzzle: Shelf order + Strips on the books.
Thought I'd put this here cause I saw one comment saying the puzzle is not about the shelves and spent 5 minutes trying to figure it out when I realized that was false. Makes sense once you understand it, but the comments led me a tad astray.
Encountering a massive bug on level 14 where the pieces are glitching and getting stuck inside of each other. When I try to move them they glitch more and start jittering uncontrollably. It got to the point that one of the pieces ended up glitching through all of the other pieces and found itself below the puzzle.
Very well crafted game. The controls sometimes feel slow, but they are actually very precise all things considered. The levels are trail and error most the time, which I feel is not a weakness here. You slowly learn to predict and manipulate the killer plumber so you can kill him. The ONLY level I did not feel this way with was level 47. It was obviously random where everything is placed each time it loads, making it impossible to predict. You literally wait for the small window to zip towards him with the ghost, and hope nothing gets in the way. And hope that you don't mess up at all so you survive to that point.
That said, I still love the rest. Easily 5+/5.
 "Forgotten Hill Pico"){kind=icon}
 "Interlocked"){kind=icon}
 "Kill the Plumber"){kind=icon}