5. How Can Smith Meet Tennant?
This question came up the second we learnt that Tennant was coming back. Previous multi-Doctor stories have always managed some strange explanation and just the idea of Tennant and Smith teaming up was enough to make everyone giddy with excitement but the fact remains. The Doctor shouldn't be meeting his former self. The closest we have to a clue from the trailers is the above shot of Smith throwing his fez into some kind of vortex looking thing saying "I remember this" followed by a doubtful "Almost remember". Smith could be changing the time line at this point, flinging his fez backwards in time to interrupt Tennant and provide a physical link for Smith's Tardis to lock-on to. This still doesn't explain why the vortex exists in the first place, but judging from the still it stems from what's going on at the National Gallery (because of the clean and empty space behind Smith), one of the few places we know will be featured but we'll have to wait to find out why.