Someone mentioned this on IRC but didn't seem inclined to actually file a bug report, so I'm doing it instead.
If you use a planetary invasion tactic like Mass Drivers, it will severely wipe out the planet's soldier population, but if you don't actually win, the planet PQ won't be reduced.
This leads to exploits like sending a troop transport with 1 person, Mass Drivering the planet's population to almost nothing, then follow up with a traditional invasion with your real troop transport.
Tada, you've just conquered the planet with minimal troop loss, and no PQ loss to boot.