i didnt think stacking mattered, its just the same thing as spreading them out, its the amount of ones you got firiring.
Anyway this happens on the least few levels on my map, especially the last level, which sucks. The last few levels every one has a ton of turrets, and you can buy spore colonies and photon cannons too, and there are 90 units being made at the start. And then the last level has 90 units being made, and then 5 seconds into the level you hear a "Finish Him" Sound and 90 more units are made, and i guess the game cant handle it. I think if i made less turrets, but more powerful turrets it would work. Got to think it over to make sure it is balanced still.