Got to end-game territory. Had 20-30 odd planets and four sizable fleets running around. There was only one AI player left and they had a huge fleet stationed at one planet. 

Took one of his remaining planets and I couldn't build anything. No ships, or orbital structures, nothing. Even when the planet was fully upgraded, I couldn't queue anything to be built. All I could see was a red message "Already a unit limit for Empire"

By this time a couple of my fleets where in position for a final assault on the huge fleet. I approached from two different directions. My two fleets combined gave me about a 2-1 advantage in ships. I also had 7-8 capital ships and they had none. Proceeded to lose both my fleets to an absolute swarm of strike aircraft. There were hundreds and hundreds of them.

No problem I thought, I can rebuild easily enough. But no, even though I had close to 900 fleet supply available I couldn't build anything, anywhere. Ended up throwing all my remaining ships into the grinder. losing them all. Only then could I construct another strike craft heavy fleet to take out their fleet.

I have a save game that I can shoot through to look at.