Player skulls can be used to pull ip's and implant viruses

If you make a player skin, you can use a decoder to change the skin to an ID. Since the Minecraft client downloads the skin without hesitation, you can implant viruses into the skin.

This means when a player uses /give to give them the skull, if a player does so much as look at the skull on a server, it can do things such as get a player's IP address, find where they are from, or inject a virus into their computer.
This exists in any MC versions 1.7.10 and above (including 1.8.4)
A video about it: http://youtu.be/EO6VXy_4y1Y