It seems that having too much production on a planet can be inefficient. If you build a ship with a cost of 100bc on a planet that has 500mp on military production, you still pay the the entire 500bc in spending. Anything over the cost is wasted bc. I've tested this with military spending. The costs for social improvements are hidden (though they could be looked up in the xml). I'm guessing that extra social production is wasted just like military, but I can't confirm it. Research does seem to rollover as I've discovered multiple techs in the same turn and the time to complete bar is usually filled some. Research will roll over into the next tech along the line unless you are at the end of a branch; In which case, it does not roll over and is wasted. Research costs also hidden (again they could probably be looked up), so I don't know if all of the extra is passed on or only some is.
Just something to consider when you are creating manufacturing planets.
