Happened once to me, too, after i played too much with graves and coffins.
Hm, that didn't sound that good. Well, i basically freed a grave that was already claimed and had some body parts of dwarf A, and the other body parts that were transported ended in another grave, sharing it with dwarf B. After i freed that grave, dwarf A remains just lied there without anyone touching them.
Just engrave a slab to fix it.