Final result: Still don't really know what it is. Brilliant.
The good news is I have narrowed it down to a couple of reasons. I'm pretty sure this happens when a group of objects is selected and applied a shader in a specific layer when some of those objects are missing from the layer.
This strange little glitch also seemed to happen when selecting a group of objects from one layer and creating a new layer with the selected objects. This obviously doesn't sound right and might just be a bug with my Maya.
If anyone else out there has had this problem please comment and let me know what happened :)
and again ....
and again ...