fall between two stools

beast with two backs

Two people united in sexual intercourse in the missionary position.

eat for two

To be pregnant

fall apart

To be emotionally in crisis.

fall around

To fall over continually, especially through drunkenness, clumsiness, or incompetence.

fall asleep

To be affected by paresthesia; to go numb.

fall at the last hurdle

To fail near the end of an activity, a project, etc.

fall behind

To be late (for a regular event)

fall between the cracks

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.