in fact id happened to me often, especially in the beginning of my experiences with FE2 on good old CD32, but since a long time a cargo bay life support is something i fit as soon as i have left just enough space to fit one and still have left a few tonnes capacity. it's, especially for a imperial commander, a very important equipment, cause it's the common way to make some extra money while you're traveling from facece to any target, destination for your duty runs, especially in the beginning of your career there. but i guess i t doesn't matter imperial or federal career i choose, even as a freelancer for corporated systems, the cargo bay life support stays with me, must be a long time since i forgot to fit one, but yes indeed not so long ago i do forgot to, fortunately in FFE, so i don't take this to hard, it simply didn't counts and i didn't only forgot to mount the life support i also forgot that i had bought some live animals or slaves, i can't tell what is was, i only remember that i was surprised i have bought fertilizer, until i recognized it must have been something that was alive before. usually i didn't fly huge ships like boa's or such, so i don't carry much goods around in a military career, i'm not a trader. and you see my ship is a constrictor there's not much left for other things than fuel on it, especially with a military drive you will allways have to carry some extra fuel.
that could be one of my quotes after 2 sucsessful reconaissance, one bombing mission while trying to leave the last system after finishing all, only to find out i have no fuel left to get home and no base or no military fuel is there to buy: "@#!!!?^-^§§%/=!!!$" "next time i won't forget to fill up my tanks to the limit" similar situation, i enter a system for such a mission and it's one of them "400AU to go" ones and i have left only one tonne of fuel,
ok you can manage to get out of that stupid situation, by accel. very slow only a few hundret km/s turn off engines and wait, wait, wait, but maybe it took you many months, so your mission is lost anyway.