Console gets spammed by this message:
http://pastebin.com/CcjzkgAS

Sadly I'm not able to tell when this happens but I try to find it out.

Caused by skeletons shooting at other skeletons confirmed.

Will be testing to see if shulkers can cause it, or skeletons shooting other mobs.

Removing any instances of drops by entities will fix the issue. Blocks work fine.