In one game, I began production of a Technology capitol, as an upgrade to a full planet. In a second game, I began production of a Manufacturing capitol as an upgrade to a full planet, which had a bunch of factory upgrades in queue.

In both cases, I cancelled the upgrade, because I wanted to locate the capitol on a different world. The upgrade was removed from the build queue, but the ability to build the Tech/Production Capitol was not restored to the list of things I can build.

It became impossible for me to build those capitols, because I had cancelled their production.