"Caves of Androzani."Emh said:
Just because they can do that doesn't mean they should. I think it would be far more fascinating to see The Doctor face his mortality when he's on his final incarnation and how he finally faces death.

No, seriously. "Caves of Androzani."
"The Brain of Morbius" shows that there are pre-Hartnell Doctors. Assuming there are no other Doctors than the ones we're shown in that episode, then Davison is the thirteenth Doctor. Which makes his final lines, "Is this death?" a reference to his uncertainty as to whether or not he can regenerate again.
Could that be why Colin Baker's Doctor is so unstable? Because he's a Doctor too far?
Intriguingly, Colin Baker's first story involves a Time Lord attempting to regenerate into his fourteenth incarnation. Coincidence?