fall between the cracks
Also, fall through the cracks or between two stools. Be neglected or overlooked; also, not fit either of two alternatives. For example, Please make sure that either our department or yours deals with this account, lest it fall between the cracks, or Trying to be both teacher and parent, she fell between two stools. The variant using stools, with its image of a person falling to the ground between two chairs instead of sitting down on one or the other, was already a proverb in ancient times; in English it was first recorded about 1390.
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