Bug 1 Killing one in the stack sometimes makes the entire stack dissapear. I think this happens if you kill one right as another one is being added to the stack. Bug 2 I think if you kill the stack too quickly (Rapidly 1-shotting them), you get significantly less xp than you should be getting. Bug 3 Stacks can phase through certain blocks as you are killing them and go to places they shouldn't be going. Examples of blocks would be hoppers and iron fences.