5. The Big Bang
Skipping ahead to the finale of series six, The Big Bang is the episode in which Amelia Pond (Karen Gillian) is released, alive, from the Pandorica after 2,000 years trapped inside, Rory (Arthur Darvill) is given the label of the Last Centurion, and the Doctor (Matt Smith) enters a cracks in time in order to close them all and save the universe. In the restored present day, Amy remembers the Doctor on hers and Rory's wedding day with a gentle nudge from River Song (Alex Kingston), and pulls him back into reality.
When He Should Have Regenerated: At one point in the episode, The Doctor is shot by a Dalek, and uses a time manipulator to go back in time to warn his previous self to create a distraction. While the younger self goes to the roof and gets shot (the distraction), the slightly older, dying Doctor makes his way to the Pandorica where, after a few tearful good-byes, he flies into the heart of the exploding TARDIS and becomes the explosion that saves the universe Big Bang Two. The problem here is that being shot by a Dalek is always immediately fatal it has been depicted this way ever since the first Dalek serial in 1963, which took place on their home planet of Skaro before they could even exist in other environments or levitate. In series one when the Ninth Doctor meets a trapped Dalek, he immediately assumes that being shot by it will kill him, until he realizes that it is truly trapped, and drained of energy. The Dalek in this episode had its energy restored by the power of the Pandorica and, while sluggish at first, gave every indication (via the iconic skeleton CG that shows up on impact) that it was up to full power by the time the Doctor is shot by it. Therefore, he should have regenerated before he could ever transport himself back in time to warn himself to create a distraction. This, admittedly, might have created problems for the other characters and saving the day, but if this is how Steven Moffat wanted to portray it, a gunshot wound would have been more believable.