Mesh Mirror - Confusing

[jaimi]

I create a mesh from some cubes (moving them around, etc).

I combine these meshes into my new object.

Then, I select Mesh/Mirror - Expecting it to mirror the mesh.  What it does, however, is exceedingly strange.

The mesh disappears. Except for a single face. (I was not in face mode, I was in object mode, and the object was selected).

I duplicate the mesh. Then mirror the duplicate. And it works. 

This is strange behavior.