I was playing on a large map against all opponents at a normal difficulty level. I happened to be doing really well for myself and had just forced the Iconian refuge into giving me nearly all of their techs in exchange for peace (poor souls thought I'd be a push over when they declared war on me).
Then they discover an ancient artifact on one of their planets bordering my space. This artifact raised all lass 0 planets within 2 sectors of that world to class 12s.
At first, I hoped I would get a few new planets to play with.... I didn't quite realise how much territory 2 sectors truly encompass.
I ended up with roughly 20 planets affected merely within my own borders. There were dozens more inside the surrounding empires territories.
Of course, I panicked and started building colony ships as fast as I could build them. At least until I noticed that none of the other empires were doing the same. I quit the game a few turns after the event so I don't know if they were all just casually buiding them or not. I realised I REALLY didn't want all those extra planets as I was perfectly happy with what I had. To get such a large amount of planets would have destroyed the game at that point.
Perhaps this event could stand a little nerfing, 2 sectors is a huge area on all but the two largest map sizes. As for me, I'll be loading up an earlier save game and hoping that this event doesn't come about again.