No more spoiler warnings. You’re on your own.
Someone pointed this out (cannot remember who) and I wanted to work it out in a bit more detail.
Captain Jack Harkness had a Vortex Manipulator.
River Song acquired a Vortex Manipulator from Dorium Maldovar in 5145.
Dorium Maldovar acquired it from the wrist of a Time Agent.
Jack was a time agent in 51st century and could have slipped into that life again when he reached that point in the timeline.
At some point, Jack becomes the Face of Boe (a living head in a jar).
Dorium Maldovar is/was a friend of The Headless Monks.
The Headless Monks behead you alive and both the body and the head continue to live.
What if the Vortex Manipulator which River acquired is actually the one that used to belong to Jack, which Dorium acquired when he arranged for the Headless Monks to behead Jack, thus creating The Face of Boe?
Bearing in mind that Moffat created all of the above characters, and his fondness for folding in his characters’ lives over and through each other, this is the kind of thing that makes an odd sort of sense.
(And yes, this scenario also has several “Rule 34″ possibilities, but I’ll leave those as an exercise for the reader.)