So, I've never really thought of this as a possibility before, but I'm sure a lot of you have and something tells me there may also be a 'Tip' that I've managed to miss all this time that addresses this, but it seems as if doors don't interact properly with sub-screen boundaries in Lunar Magic.
I only discovered this because I received a piece of feedback in my hack stating that a door in one of my levels would sometimes act as a reset door (as it was supposed to), but other times it would send the player to the endless bonus room (and you could test this with savestates even, not just reloading the level by dying).
I wanted to investigate why this was and found that the door in question was directly next to a sub-screen boundary (touching it). None of the other doors in the area were touching, and none of them had problems. And sure enough, once I moved the door a mere one tile away from the boundary, the problem stopped.
Has anyone else encountered this? Is this well documented? If not it might be worth mentioning in tutorials because it's a major bug that can be tough to catch because it only sent me to the endless bonus once every 10 or 15 savestate loads.
This has to do with the fact that a screen exit checks for Mario's position i.e. Mario's top left corner. That means, if the player isn't entering the door from the right side, Mario's position is actually outside of the block (it happens because the block hitbox is made out of interaction points and the doors uses the one in Mario's center) and therefore might end up on a different subscreen if the door in question is on a boundary. That being said, because Mario's position is on the top left corner, this only happens if the subscreen boundary is to the left of the door. If the boundary is to the right of a door, it always works fine.
Regardless of which solution you choose to do, I definitely think this should be added to the 'Tip' bar if it isn't already. Because as I said it's sort of a tricky thing to catch if you aren't aware of its existence already.