Howling:Regeneration in The Impossible Astronaut

The Impossible Astronaut starts with the Doctor's "death." After River shoots him, he starts to regenerate, but river shoots him again in the middle of his regeneration cycle, killing him. Why does he start to regenerate? As shown in The Time of the Doctor, he has no regenerations left. Was this a mistake on Moffat's part? Or was it because the Doctor's death was supposed to be as realistic as possible, and everyone present was unaware of the war doctor? CloneMarshalCommanderCody ☎  14:06, August 19, 2014 (UTC)