If you box select a note and delete it by pressing backspace while still holding the left mouse button, it will delete either the next note after the note you box selected, the previous note before the note you box selected or the note you box selected. In any case the note after the note you box selected will light up and will stay lighted up if you don’t move your mouse
If you continue pressing backspace (while still holding the left mouse button and without moving the mouse) the note that is deleted will be the note before the note that was deleted before, and the note that is lighted up will change to the next note after that. If you keep pressing it until there is no note after the current note that is lighted up then hookpad crashes (this assumes that there are notes before the previous deleted note that can be deleted)
This can happen in any hookpad project and can also happen with chords if you box select in the note staff and expand the selection into the chord staff