The action parameter changed from a direct GameObject reference to a variable name? Or a variable of None?
It was a direct GameObject ref that changed to an fsmVariable (gameObject) that was being used in the same state on a different action (and other states throughout the fsm). At one point I had the (disconnected after applying) scene hierarchy in the way it was made while its prefab had this altered variable on a particular action (get fsmGameObject).
Unfortunatly my best guess on the workflow that sets this off is too vague to be useful, feels like its something to with working on multiple scenes with scene hierarchy's all associated (but disconected) to the same prefab.
I will try & provide you with harder evidence if/when this weirdness occurs in the future as by using the following method normal service seems to be restored.
@FritsLyn you may want to look into this...
According to the method written by ferraribeng at the bottom of this page:
http://answers.unity3d.com/questions/56096/disconnect-prefab.html the disconnected scene hierarchy becomes disassociated with any prefab. I then dragged an empty gameobject onto the troublesome prefab (in the hopes of 'blanking' it as i want to avoid deleting it), deleted the (now blue) emptyObject in the scene then dragged my unassociated scene hierarchy back onto the prefab.
Now this seems to be working for me (for now) but, your mileage (and mine) may vary.