It means that something that normally would be regulated or counted as part of some sort of procedure was missed and was not

I guess normally it’s used in business and government, a good way to think of it is since gov and business normally have to make decisions that affect ‘most’ people but not ‘all’ people, sometimes things and people get left behind. When this happens we can they slipped through the cracks. So I’m general it’s something that is forgotten, but not intentionally. https://idioms.thefreedictionary.com/slip+through+the+cracks