Somehow the appearance of one of my faces was changed, and I cannot delete the appearance.  According to the hierarchy of appearances, faces override features, features override bodies, and bodies override parts (additionally, assembly appearances override any appearances at the part level).

So, I have a face that has a particular appearance.  It cannot be changed unless I select it individually, or override it at the assembly level.

In the above image, there should be a red X next to "Face<1>@...", but there is not.  When I've added appearances to other faces, there has always been a red X.  I will attach the part.  Originally, the part was in an assembly, but I have broken all the links thinking that maybe that had something to do with it.

Someone work their magic!