So, Series 7 begins with a shooting death.
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At a picnic in Utah, the Doctor is shot by a figure in an astronaut suit. As his body begins to show the golden glow of regeneration, he’s shot again, disrupting the regeneration and permanently (!!) killing him.
Moments later, the Doctor shows up: a three hundred year younger version of himself. The remaining episodes of Series 7 deal, in some measure, with his companions agonizing over the fact that they know the date, time, and place of his fated death, but can’t tell him, lest space and time are disrupted by paradox and get even more wibbly wobbly timey and whimey.
Later on, we learn that this incarnation, the Eleventh Doctor, is in fact the last of his available regenerations. The Eighth Doctor regenerated into the unnumbered War Doctor, so that the Ninth Doctor is really number 10. And the Tenth Doctor halted his own regeneration once, pouring the energy into his own extra cut-off hand, setting up the biological meta-crisis that created the human version of the Doctor that lived happily ever-after with Rose. So when Eleven came along, he was really 13.
Therefore, when Eleven was shot by the astronaut, he should not have regenerated.
But of course we also learn that what was shot was NOT The Doctor, but rather the Teselecta, a robot controlled by miniaturized people, one of whom was the Doctor. He faked his own death by allowing the robot to be shot and cremated. Since the Teselecta robot was just a robot, The Doctor must have added a fake “regeneration glow” effect to happen when the robot was shot.
But why did he do that, when in fact if he really had been shot, there would have been no regeneration?