This has only happened to me once, but one time I was cutting into a chunk of tar heroin, about a quarter gram-sized piece. I was using a knife if I remember correctly...anyway, for those who are unfamiliar with tar, it comes in a variety of consistencies, as far as firmness is concerned. Some tar is soft and gooey, while other tar is hard and brittle. This was hard and brittle tar. Usually I would breathe through a straw unto it so the moisture of my breath would make it a bit softer and easier to separate (or if I'm going to "tar up" gunpowder heroin...old junkie tricks), but for some reason this time I didn't.
Anyway, the piece separated in two, with one piece literally flying through the air and hitting several objects in my room with slightly audible ping!-sounds before landing SOMEWHERE. Me and the chick who was over at my house at that time tore my room apart looking for that piece of dope, which was only slightly over a point but we were damned if we were gonna lose a point of dope, garsh darnit! We re-arranged furniture and got down on our hands and knees to carpet-surf for that chunk o' goodness...it came from a batch of good tar, too, so I was loathe to stop looking for it. Eventually we called off the search, though.
I found it the next day. I noticed a black piece of something right behind the leg of my sofa, and thought, "could it be? Is god smiling upon me today?" and lo and behold, it was tar! It was like I could hear angelic harps in the background and a ray of light me as I gazed in wonder on my find.