Production should overflow
As the title says. Even in a game where you can micromanage each individual city/planet, the idea of micromanaging to avoid wasted production is antiquated. In a game where you set sliders at a macro level and hope you aren't wasting too much? Crazy. I've tested this and it doesn't overflow for ships or improvements. You also still PAY for the production that is wasted! This is a mind boggling design decision that should be changed with the next patch.