Imagine a cubic volume of space, defined today, with edges of the order of the Planck length. As cosmological time goes forward this volume expands so that the edges of the cube expand with the scale factor. This seems ok. However, as physics is time-reversible, it should be valid to imagine time going backwards so that the edges contract with the scale factor. But then they would be smaller than the Planck length. If the Planck length is the smallest length scale then is there a contradiction here?