I have seen it, but it has been almost forever. Probably ten years.

Given timey wimeyness, I almost wonder if the White Guardian is Thirteen. Or some higher number...it is, after all, possible that the limitation to twelve regenerations was due to division of available energy among many Timelords. Uh oh, going over speculative again.

You think Eleven will wake up with a power allergy after almost having gone too far? I guess Moffat will let us know.