In the list of possible heads to drop, they include some legendary names, including notch, etc. It's possible Aikar accidentally/on-purpose included the heads in the possible drops. It's also possible he gave the id for a skele head to the same id Herobine head uses and the wrong one dropped. I think it's most likely an ID mixup. And if it's fixed, you might have a very unique head. You should hold onto it for a while until it becomes a unique collectable.
Quite recently in game there has been heads popping out of things... It may seem strange but it's true. I was harvesting a little patch of sugarcane and 2 player heads popped out and I picked them up. I can remember the names of the 2 but I can remember 1 off the top of my head. One was ElectroBombs head and can't remember the other. About 2 days ago I also heard a person telling us that a head popped out of something else. Maybe it's a bug...
I know someone else who was harvesting dark oak in town, and on multiple occasions got some player heads. But that was a few weeks ago.
