Found an interesting interaction with Mind Controlling Darkcasters and Priests inside Mana-Tombs while levelling my Priest.
Upon using the Power Word: Shield on the mind controlled priest I was then locked out from using it myself. Reloading UI did not fix the issue but resolved itself on death.
Found this interaction curious so tried on the darkcasters and once I had used the Shadow Word: Pain I was locked out from using.
I could cast these spells from the spellbook. Both of these spells on my bars are in macro form. So I think the problem lay there.