I had 2 colony ships in orbit around a Yor world, one purchased, one built with the purpose of loading up on colonists.   The first ship took the 0.5 Million of population that the world had, leaving nothing for the second colony ship.

When the second colony ship tried to launch, It asked me how many colonists I would load up and 0.5 was the minimum answer.   I didn't have 0.5 million so I hit cancel.  The cancel did nothing (it should abort the launch or accept zero as a valid number).   I might want to launch it with 0 in order to upgrade the ship to something else like a constructor.

As it was the dialog box kept re-appearing and I had no way to exit except by closing the game, restarting it and decommisionning my extra colony ship.