Bug: It is uncertain exactly what causes this, but sometimes, usually with bats, the quest counters for killing flying enemies won't tick up when killing them off screen with arrows. However, if you slow down a bit and let them appear on screen, then kill them, once you start shooting arrows off screen again, the quest count will start to tick up again, killing them off screen.
Character mistake: Once you unlock the quests to track down Victor in his house, you have to switch to the character Anna in order to progress. Once you get into his house, you eavesdrop on Victor's conversation with a strange stone, and it comes up that one of the slayers is a young lady with brown hair and green eyes. This is referring to Anna, and you have to be playing as her to do this quest and interact. However, if you're using the Bunny Outfit on Anna, her hair is blonde, but you can still do the quest.
Trivia: There's a secret on Bonus Stage 2. If you time the jumps right on the second part of the stage and go up high on top of the cave instead of going through it, there is a black hand-shaped monster. It doesn't do anything, but once you find it, you will never be able to find it again as it only appears once. And once you restart the game, you will be met with new title screen music.