The Howling → Timelord Biological Metacrisis
The Doctor and River Song both made it quite apparent that excess regeneration energy left in the few hours in their post-regeneration states allow them to regenerate from at least a reasonable amount of injury at what appears to be no cost to them.
Given the number of Gallifreyan villains who has appeared, if the metacrisis cloning process done by the 10th does not cost a regeneration. This mostly harmless technique would allow them extra regenerations simply by cutting off parts of themselves to carry around right after a regeneration and use them at their next regeneration.
The implications seem to suggest that the Meta-Crisis Doctor did cost a regeneration.--218.189.39.199talk to me 09:21, August 26, 2012 (UTC)