10 Doctor Who Moments Deeper Than You Think
7. The Leftover Squareness Gun
When Steven Moffat introduced River Song in Silence in the Library, there were wild theories over who she actually was, as if the mere revelation that the Doctor had a canonical wife wasn't enough!
The space archaeologist had a lot of similar tech to Captain Jack Harkness throughout her Doctor Who appearances, including a vortex manipulator.
While being attacked by the Vashta Nerada, River produces a squareness gun, which allows everyone to escape through a gap in the wall.
This was similar to how Captain Jack helped the Ninth Doctor and Rose escape the gas mask zombies in The Doctor Dances. Did this mean that River was actually Jack? No, it meant that, because the TARDIS flew off and left the resurrected Jack behind on the Game Station, all his stuff was still onboard.
When Jack and the Doctor did reunite, he didn't even get in through the door, forced to cling on for dear life (lives?) instead.
So, at some point in the future (perhaps while traveling with the Eleventh Doctor), River borrowed Jack's squareness gun, just in case it ever became useful.