Academy's Seven Mysteries Murder Case: File 3 - S1-E3
Plot hole: A corpse has been buried inside the walls of the building for 10 years. It's covered by just a poster, and a poster from a current high schooler. If we assume the first victim was the one who put the poster there, then it means that it was just randomly uncovered by the earthquake from episode 1, which would make the chain of coincidences nothing short than miraculous. She basically one day said "I heard there was a murder here where we all hang out all the time" and the ground by its own accord opened to reveal a skeleton without her doing anything.
