I placed a torch on the side of another torch while there was a block nearby, and I got a floating torch. This happened on accident while I was messing around in survival mode, and I decided to test it in creative for a bit.
After testing for quite some time, I found out that giving the floating torch a block update would cause it to pop back into an item. I also did a line-up of several different circumstances where this might occur.
(Edit, ET 9 minutes after posting): I think that this bug may already have a post or two, but redundancy is a good thing, is it not?
Indeed already posted. MC-57714