Under certain circumstances you could give planets to an opponent with spore ships waiting next to them. Then declare War and retake those planets on the same turn thus stealing loads of technology from them.

Buildings stay intact only down side would be loss of population on those planets but you could easily do this on low pop worlds or offload population using transports then move the population back on once you've used the spore ship.

You could gain huge amount of tech using this very cheesy tactic eg easy to get that doomray you always wanted.