As you can tell from the photo above, A great deal of care and hard work was put into creating this retaining wall. It's an excellent example of a hybrid, using stone, mortar, and brick. What appears to be a concrete finial tops the column that stands at the base of the stairway.
Unfortunatly, there are numerous examples of hybrid retaining walls in which the second material was added as a repair! But I'll leave some examples of those in the "Hall of Shame" section. My own back yard has one hybrid retaining wall in which concrete slab chunks, probably from a sidewalk, have been stacked up to form the bulk of the wall. A fence was put on top of this, but to properly anchor a fence metal plates should be set in cast concrete. Thus a thin strip of concrete was cast on top of the slabs. This poses a bit of a problem for me, as my yard is on the higher ground. I have to access my neighbor's yard in order to clean out invasive English Ivy, whose roots can destroy or dislodge the underlying concrete slabs. All in all, a poor design.
The worst hybrid designs I've ever seen in Seattle are cast concrete slabs which use railroad rails as "rebar" reinforcement. I know they are railroad rails and not ordinary rebar because they are on the outside of the wall! As well as being unduly ugly, the exposed steel is prone to corrosion, and acts as a public shelf for accumulated litter.