this bug is so annoying.
and its mostly related to soap.
as soon as i got soap production going i began getting this from a few animals. then a few more. then my dwarves. watching my soap stockpile, i would see horses, dogs, cats, racoons, foxes, elephants and my one lonely grizzly bear all go up, grab a bar of soap in their mouth, wander a few squares and drop it, giving me the error message.
so animals clean themselves near a water source, like dwarves do, and like dwarves animals think they need soap but somehow cant get the useage of it happening right. although after i made an indoor bath house i think some of the elephants may have been pulling it off with that trunk of theirs.
annoyingly enough, a lot of my dwarves do this as well. soap use for cleaning outside of hospitals seems to be very messed up, even without the tame animals part. trying to keep your soap and washroom behind animal frobidden doors sortof works, making the soap stockpile a hospital zone does not, although the animals dont go near my proper hospital. only real fix seems to be forbidding all soap like lordzabujca suggests, although you can try to get some into the hospital every now and again when they need it, as soap stored in boxes or bags in the hospital seems to be immune to this. just so long as its not in bins in a stockpile.
its more annoying then the "cat cancels place item in stockpile: too injured" bug of 40d, because not only does it spam you incessantly, it ties up tonnes of dwarves who have to keep returning the dropped bars of soap. and it seems to randomly happend to dwarves as well, which keeps them stuck in a little loop of constantly trying and failing to clean themselves, and leaving bars of soap everywhere just like the animals do. having both a well and a ramped pit of water next to the soap stockpile did not help.
EDIT: blood in the water source does not seem to do anything currently. in previous versions decaying bodies in the water source did not do anything bad either, and even with all the new diseases im pretty sure this still holds true.