Found 2 Skeleton Spawners. Made a Mob Trap with water.
Skeletons shoot arrows. All arrows shot into cobblestone stay forever. Water will remove them. They do not despawn however.
Step 1. Create Server.
Step 2. Find Skeleton Spawner
Step 3. Let Spawner Spawned Skeletons shoot Cobblestone (Or any other block from what I can tell)
Step 4. Watch Entity Count Rise to unacceptable levels.
Duplicate of MC-31577 - If you have not, please use the search function in the future, to see if your bug has already been submitted. If you could not find the original report, please comment with the keywords you searched for.