Bukkit

What steps will reproduce the problem?
1.when skeleton shoots arrow
2.when you attack monster sometimes
3.

What is the expected output? What do you see instead?
expected: no error, no lag. I see errors and lag

What version of the product are you using?

latest
Do you have an error log of what happened?
server couldnt add event to event org.bukkit.event.entity.Entitydamagebyentityevent to Achievement

Please provide any additional information below.