A Spondylolisthesis, or slip between two vertebrae, is a common problem and there are several reasons why it can occur.
The scan above is slice through the middle of someones spine, it shows a slip between L5 and the sacrum.
The diagram above shows the yellow nerves being squashed as a result of the slip. Spondylolisthesis can cause a spinal stenosis
This slice is taken nearer the edge of the spine where the branches of the sciatic nerve, nerve roots, are coming out though holes in the spine...The bottom nerve, L5, has a very narrow hole because of the slip. It is often pain from the pressure on L5 that causes the most pain.
This example demonstrates a spondylolisthesis between L4 and L5 on an xray..you can see the holes for the nerves but cannot see the nerves themselves on an xray
On the the images below the black arrows point to a pars defect or fracture...the red arrows show the normal appearance