Actually on second thought, it should be
destroyed -= "\ref[A]"
destroyed += "\ref[A]"
destroyed["\ref[A]"] = world.time

The reason for this is that BYOND is aggressive about reusing refs. Created objects tend to wind up with recently deleted refs. If they get deleted quickly too, they can and often do wind up with the same ref as an object that's already in the destroyed list. This results in the whole list getting out of order and causing problems.