Slice-overlap artifact

The slice-overlap artifact, also known as cross-talk artifact, is a name given to the loss of signal seen in an image from a multi-angle, multi-slice acquisition, as is obtained commonly in the lumbar spine. It should not be confused with cross excitation which although similar in causation, is not due to angled images. 

If the slices obtained at different disk spaces are not parallel, then the slices may overlap. If two levels are done at the same time, e.g. L4-5 and L5-S1, then the level acquired second will include spins that have already been saturated. This causes a band of signal loss crossing horizontally in your image, usually worst posteriorly. The dark horizontal bands in the bottom of the following axial image through the lumbar spine demonstrates this artifact.

As long as the saturated area stays posterior to the spinal canal it causes no harm.