Humanity's collective efforts in space are one of those things that are pretty well celebrated. You'll never meet anyone who says "Did we land on the moon? Hm, I think so. When was that, in the 80s? I think I remember Reagan talking about it." Because the achievements are Big Deals. So what about all the great stuff that wasn't?
Picking a forgotten space milestone is tricky — my first instinct is to point to the smuggled corned beef sandwich on Gemini 3, but I think when it comes to unsung achievements, you have to look at the Soyuz T-15 mission of March 1986. This was the first mission (and I think only) to fly a single spacecraft to two space stations! The Soyuz was first launched to the newly-launched Mir core module, which they helped set up and get ready for use.
Then, they got back in their Soyuz, undocked, and travelled to Salyut 7, the previous Soviet space station, which was flying out of control and frozen. They carefully and manually docked to the uncooperative station, and brought it back to life, even though it was so cold inside their instruments couldn't measure it, and they had to time their spit freezing on the walls to figure it out.
They eventually got it running again, and then gathered crucial equipment and experiments and ferried them back over to the new Mir space station.
So, three space station dockings, resurrecting a frozen corpse of a space station, and lots of inter-orbital navigation and travel. We should all know about that mission.
So, come on pals, beat that.
What's The Greatest Space Achievement Nobody Knows About?