Revealing mistake: When the Cyberman arm is shooting at the Doctor and Amy, just after the Doctor jumps out to draw its fire, Amy leans on the Pandorica and you can see it bend in, revealing that it's made out of plastic or something. (00:20:42)
Revealing mistake: As Amy stands where she is told, a cable comes towards her feet. As it does, a black thread can be seen pulling it up to wrap around her leg. (00:21:39)
Chosen answer: 1) When Stephen Moffat took over he ignored a lot of what had been developed before (there is not in-universe answer). 2) It would have killed Rose, so the Doctor absorbed the energy. His body regenerated before the energy could do a significant amount of damage that would prevent regeneration.