Put the summary of the bug you're having here
I was building a skelleton xp-farm. When using it I suddenly had 4.7k entities shown at f3 and only 0-1fps.
(both gamemodes)
The skellis spawn and fall down to the stoneslab.
http://imgur.com/fP6bZlb When you stand under the cobblestone the skelletons start to shoot thousands of arrows that get stuck in the cobblestone and dont disappear like normal arrows. You have to break the cobblestone so the stuck arrows fall down. Then they disappear. after 1-5mins.

EDIT: http://imgur.com/44dLjbH made this in vanilla minecraft. It happend on the server aswell. (look at the entities)