I played FreeSpace 2: Colossus quite a while ago, and I remember one mission in particular. The mission involves (not surprisingly) some dogfighting. However, partway through the mission, an enormous enemy vessel shows up, and you're ordered to retreat as fast as you can.
However, if you stay behind and defeat the remaining enemy dogfighters, all becomes quiet. The large ship doesn't actually attack you, nor are more fighters released. In fact, by sitting there with my fighter's weapons firing for around half an hour, I remember that I was able to lower this enemy giant's health(?) down to the 20% range.
What happens if you take the time to lower the health of this ship to 0%? Is it even possible? You're certainly not supposed to even attempt to engage with this particular ship, at least not at this point in the game.
Edit: I think this is the enemy ship I'm thinking of: