The Doctor and Susan are about to steal a Tardis and flee Gallifrey.
Until Clara stops him, and tell's him to take the one with the faulty navigation - but at least it would be so much FUN !!!!!!
This kinda changes things I think, cos we've always believed that the Doctor stole the Tardis with Susan, and in 'The Doctor's Wife', the Tardis herself said "You stole me, and I stole you", but assuming that Clara [or at least a Gallifreyan version of her] was on Gallifrey the day the Doctor fled his homeworld with Susan, then it seems he only took the Tardis we know to be his because Clara told him to take it, cos it seems he would have gone for another one and not the one we know to be his.
Funny thing is, how did she know the Tardis she was standing next too would eventually be the one he would end up with as a Police Box ?, cos it shown with other Tardis's, all un-disguised and identical.
I'm not gonna think about it. I'm going to bed . . . . night night, Zzzzzzzzzzzzzzzzzzzzzzzzzzz