Sometimes the bottom vertebra of the spine (L5) or the top of the Sacrum (S1) forms slightly different than usual.
If the L5 decides it wants to be part of the sacrum it's called "sacralization" and when the S1 wants to be more like the lumbar spine it's called "lumbarization".
These changes happen in about 10% of the population and most of the time they don't cause any problems for the spine. But recent research may be pointing a different way.
In a recent study that involved 1,875 young males with low back pain (LBP), researchers observed that 32% of the participants had a lumbosacral transitional vertebra and that the most common subtype of transitional vertebra was associated both disk and facet degeneration.

Source: La Radiologia Medica, 2018.