Following up on my item a couple of weeks ago, Death of the Doctor has now broadcast, and perhaps inevitably fandom has gone into overload over the "507" comment. Russell knew this when he wrote it, something he acknowledges in an interview with SFX.

Of course, people seem to have missed what was actually said. Clyde asks how many times he can change his appearance, and the Doctor answers "507". Not how often can he regenerate - that word doesn't even crop out in the entire story! And we have two precedents for this in the classic series.

1. the obvious Romana regeneration scene in Destiny of the Daleks, where she tries on several bodies before settling on the one she wishes to 'wear', i.e. Princess Astra (indeed, she chooses this twice in the story!). There's no inference that changing appearance in this way has any detrimental effect on future regeneration.
2. the slightly less obvious scene in The Brain of Morbius when the Doctor and Morbius push each other back through their past. There are several faces attributed to the Doctor himself that we've never encountered before, but again this doesn't seem to affected the regeneration count either.

So maybe Time Lords are limited in regenerations but not quite so limited in their appearance after all.

But then again what would Doctor Who be without contradictions for us fans to argue over?!!