Fall between / through / the cracks / two stools

fall/slip through the cracks

to get lost or be forgotten, especially within a system

'It seems that important information given to the police may have fallen through the cracks'

See more at http://idioms.thefreedictionary.com/fall+through+the+cracks

fall between the cracks / fall through the cracks

to fail to fit into a given agenda or program

'Some things fall between the cracks - like the value of an anaerobic adhesive at GE'

See more at http://www.collinsdictionary.com/dictionary/american/fall-between-the-cracks

 

fall between two stools

to come somewhere between two possibilities and so fail to meet the requirements of either.

'The material is not suitable for an academic book or for a popular one. It falls between two stools. He tries to be both teacher and friend, but falls between two stools.'

See more at http://idioms.thefreedictionary.com/fall+between+two+stools

 

the idea that anything can “fall between the cracks” just doesn’t make sense to me.

I picture two parallel cracks. Wouldn’t the space between them be the surface?

My guess is that the current illogical form came from a blending of the established metaphors “fall through the cracks” (as small objects might fall through the gaps between floorboards) and “fall between the stools”

See more at http://www.word-detective.com/2008/11/fall-between-the-cracks/