God forbid you also put a flat sheet in. You're better off just throwing the whole tangled mess in the trash, washing machine included.
idk they kind of still do that by themselves, maybe wash them all at once if you have enough for a load
Washing bags exist, usually used for fragile/dangerous (zippers) clothing pieces, but you could surely get them in sheet size too.
Why was my first thought upon reading "Duvet Cover" that someone had made another version of "Serial Experiment Lain"'s opening (-_-)
I don't even watch that much anime any more, yet the weebness won't evacuate my mind...