This question already has an answer here:
In The Day of the Doctor, all 13 Doctors turn up to help save Gallifrey, and it's assumed that Ten, Eleven, and War have summoned the rest. Given Twelve's presence - a form that Eleven hasn't yet seen - how come Eleven doesn't know he's going to regenerate again? In The Time of the Doctor, he tells Clara very clearly that he can't.
Two possible answers that don't quite work:
Twelve wasn't summoned; he turned up of his own accord (because Eleven's experiences are in his past but in the future of all the others). But if he remembered what really happened that day, then Eleven should have remembered too; also, even if he wasn't summoned, the Gallifreyan defence council saw him, and Eleven should have made the obvious deduction from the Time Lord General's exclamation "thirteen!"
Eleven lost some of his memories of the end of the Time War, due to being together with too many versions of himself (Ten and War definitely forgot everything that happened, because 'the time streams were out of sync' or some other wibbly-wobbly timey-wimey stuff). But Clara should have remembered Twelve's appearance, since she was presumably with Eleven in his Tardis and no older version of herself was around, and her only rebuttal of Eleven's defeatist I-can't-regenerate I'll-die-on-Trenzalore speech is "you can change the future" rather than "we already know you'll regenerate again".
Why do Eleven and Clara both seem to forget having (summoned and) seen Twelve?