I'm guessing here....
You can rush buy a building. If you switch 1/2 way thru construction, your build potential does not switch to the new building. Military projects (ships) the potential does switch.
So the way the game is currently set up, you can build multiple social (building) projects due to the fact that the potential is not transfered, but it stays with the building that was being constructed. Military projects it does transfer, so if constructing a battleship and then switching to a defender, you will get a defender each turn until the potential you have accumilated runs out.
I don't think this is a bug. This is an exploit (that I suspect will be changed to only allow one purchase per turn).