ORIGINAL: paulderynck
Nothing really - once it's at zero or worse the production is zero. But it can get to worse via the "double-dip" ruling on strategic bombing that was affirmed in the FAQ.
Correct -- you can't actually produce a negative amount and neither do you have to remove anything from the production track if your production points are negative -- you just produce nothing.
How the double dip works is this:
* X has 5 resources, 5 factories, therefore 5 production points.
* Y strat bombs 3 of X's factories, reducing X to 2 production points.
* Y then overruns, say, 3 of X's factories. X now has 2 factories, 5 resources, 2 production points, but has lost 3 production points to strat bombing, for a net total of -1 production point. X produces nothing.
It's also quite easy to reduce Japan to zero production or lower with the use of strat bombing. Japan starts with, say, 10 resources being convoyed via 10 convoy points to home factories for a total of 10 production points (ignoring the ones that are already there just for the purpose of illustration). The USA strat bombs 5 factories, say causing -5 production loss. The USA then sinks 5 convoy points, and Japan has run out of spare convoys so can't replace them. The end result is that Japan now has zero production despite the fact that 5 resources could get through to 5 unbombed factories. The points lost to strat bombing cause a double dip effect because Japan can't say, after the strat bombing happens "Oh well, I wasn't going to send any of my resources to that factory anyway" (if Japan could say that then strat bombing would have little or no effect because Japan could always shuffle the resources off to unbombed factories).