So, first, I've been trying to wean the babies and have persuaded them, with great reluctance, to give up their morning post-nap-pre-lunch feeding, as long as I have lunch and plenty of it ready immediately. (I feel bad about weaning them because they're clearly not eager to do so, but I'm also not eager to continue feeling like a sow. Nursing two babies is just not very cozy and nursing two squirming toddlers is not cozy at all.)
Anyway, I went in this morning and absent-mindedly sat down and started nursing them. After kicking myself for throwing out two weeks' worth of work, I set out to change their diapers. D3's was hardly wet at all, but I figured since she has a rash I might as well change it while I was at it. I set it aside and put the still-damp-from-the-dryer clean diaper on her. Then I changed D4. When I was done, I looked and realized one of the diapers left over was another damp one from the dryer--in other words, I had put D3's used diaper on D4. I'm guessing nobody else in the world has ever done this.