The bug where you set some ships to hunt pirates/protect trade, and after a while if you get into a war and they get into a sea battle and lose and flee into a port. If you don't manually micro them they attempt to sail out again straight away in spite of your ship's health settings, usually resulting in a complete stackwipe. Usually they'll have a 'fleet is staying in port because there are superior enemy forces nearby' modifier on them too, but they'll sail out anyway. Highly annoying and I've lost a fair few number of light ship fleets this way since 1.17