its possible to fall out of the screen and get soft locked on level 4 top right of the level
Viewing post in Mirage Castle DEMO3: Exploring Platforming Mechanics comments
its just after the checkpoint in the very top right of the level - you drop down onto a diagonal moving platform (that has the blue triggers on it )
if you then jump off to the left you fall through a piece of black background and disappear off the bottom of the screen ... i found it when searching for coins !