Wiktionary
vb. (context idiomatic English) To wind up in an unexpected situation which is, inadvertently, invisible to or not handled by whatever process one had hoped to be subject to; to be overlooked.
Usage examples of "fall between the cracks".
Ask the Director of Admissions if he has any applications from qualified people who would make terrific med students-but who, for one reason or another, fall between the cracks of all the scholarships and can't float a big enough loan.
Just as she was thinking that she never would quite reach it, or if she did would reach it as such an atomy that she would fall between the cracks of its paving-stones, she stopped and listened.
But right now, with all five galaxies in a state of uproar and navigation lanes falling apart, they seemed likely to fall between the cracks of bureaucracy, at Kazzkark Base.