BUG:
If you have a resolution higher than 900 x1440, you can ignore this message.
This game was built on a 1080 by 1920 monitor, other resolutions were not tested.
In order to get the secret ending, there is a certain room in the game where you need to use an enemy projectile to hurt yourself and use the invincibility frames to get further, but if your screen resolution is lower than 1440px, than you can't access it.
This is because the games windows size is based on your screen resolution and enemy projectiles only appear when they are in view. (I know... dumb programming idea.) But, if your resolution IS 1440px, then you can get the enemy bullet to spawn by just grazing the player against the coins in the middle of the room.
The room is the 2nd last room in the game.
The only work around for this is to change your screen resolution temporarily. I'll probably fix it in an update.