In the millenia that the Ecumene has existed for, there have been numerous cases of a planet's people losing space travel. This is most often by choice, though accidents occasionally occur. What unites all of these cases is that they are temporary- when a world is not abandoned (and often when it is), it rarely takes more than a couple centuries for it to reach for space as a group once more. There is one case of this that is uniquely contrived- the planet Aakernund, in sector 340. Aakernund was first settled about 100,000 years ago and habitiformed then, but the civilization there eventually packed up and left. In 3319 Aakernund was first settled from the Ecumene, and its land recieved the touch of cities once more.
However, the people settling it eventually decided to leave the planet- if only for the aesthetic value of the buildings shot through with greenery. Aakernund was settled once more from sector 343 in 6550, with the express goal of simulating the rise of starfaring civilization. Unlike other projects in the same vein, multiple groups would compete to be first to reach various milestones. This culminated in the year 6739, when five groups launched simultaneous missions to land on Aakernund's moon at the site of an ancient multi-layered base. When they landed, the truth was revealed- each of the five groups had launched a fake mission that had no crew aboard.
There was, of course, massive outcry. Of the 13 other groups still focused on eventually reaching the stars, all published an official condemnation of the five. Some groups even demanded that the five have some or all of their crews and workers redistributed, but it's generally agreed that this demand was simply an attempt to get a leg up on the more successful groups (most of the demanding groups had switched over to the long but difficult path of going from supersonic carrier aircraft to orbit-capable aircraft by this time). Each of the missions' return trips were aborted- the rushed completion of the spacecraft bodies meant there was no alternative mechanism for returning samples, so it would be better to leave them there for use as consumables depots for later expeditions. Each of the five groups took a step back to reevaluate- some of them had even kept the secret of there not being crew aboard internally.
Every mission would require at least one new rocket and spacecraft to be prepared- some prioritized spacecraft mass over single-launch ability, and as such would require two or more. The first two attempts at an actual flight were aborted in Aakernund orbit- for one, the rocket carrying the moon lander spacecraft itself had a catastrophic failure (a setback that would mean its position in the first race would never be recovered); and for the other a heat shield failure necessitated a rescue mission. As the third geared up for the third of five launches required to build its bulk out of the low-mass fuel tanks their best lifter could lift, the dark horse candidate made its move. Aakernund's seventh (or approximately seven thousandth, including previous inhabitations) space station pushed out on low-thrust engines for a week and captured into orbit around Aakernund's moon, a lander descended... and half a day later, the crew was knocking on the airlock doors of each of the other "crewed" spacecraft.