In the last moments of The Time of the Doctor, when the young man enters the tower, the Doctor says that he has a plan, and then tells Clara that actually, he has no plan.
But the city has a truth field created by Gallifrey coming through the crack in the wall.
So, how could the Doctor lie at this moment?
He didn't lie, he just used misleading word choice.
The Doctor did have a plan: he had a clear expectation of what was to come and the across he would take. That plan just happened to be bunk, but not enough bunk to be a lie when said to a child.
Clara, on the other hand, was an adult and companion, for whom the Doctor having a plan carried the very specific connotation of a way to resolve the current crisis in an acceptable fashion.
And, of course, this entire theorum assumes that (1) the truth field was still in effect after so many centuries, (2) the field was compelling truth instead of barring deception, and (3) there wasn't just a specific exclusion for children within the field.
(It'd be a pretty lousy childhood in a town called Christmas if no one could ever tell you tall tales, ghost stories, or the legend of Santa Claus.)
(Not to mention that the strength of the truth field varries by location, and long exposure seemed to reduce its efficacy.)
As I understand it, that scene is similar to the one that occurred earlier in the episode with the wooden Cyberman. It may not be possible to outright lie while within the confines of the truth field, but you can misdirect, and the Doctor is clever enough to use that to his advantage. He explains to Clara that people love it when he says he has a plan. So in saying "I have got a plan," he isn't trying to confirm whether he actually has one or not, but rather, he is trying to say that he has a plan. He isn't lying so much as he is making an empty statement, kind of like how he did indeed send the signal to the Cyberman's weapon despite knowing that it wouldn't work on its wooden technology. In this case, he does say that he has a plan, despite knowing that what he is saying doesn't technically apply to what Not-Barnable may expect it to.
We could come up with all manner of justifications--hundreds of years acclimated him to its effect, or he meant he had a plan about something else entirely--but there's no solid answer, nothing to support one theory over another.
It looks like an inconsistency in the name of a poignant joke, and I suspect that's all it really is: When Moffat has a choice between consistency and emotion, he always goes for the emotionally resonant option at the expense of logical reason.
