Bukkit

What steps will reproduce the problem?

1. Killing Monsters as intended
2.
3.

What is the expected output? What do you see instead?

Sometimes you don't gain Life back.

What version of the product are you using?

The newest.

Do you have an error log of what happened?

Zeile 17753: 2012-09-21 13:20:33 [SEVERE] Could not pass event EntityDeathEvent to LeechLife
Zeile 17708: 2012-09-21 13:20:30 [SEVERE] Could not pass event EntityDeathEvent to LeechLife
Zeile 17663: 2012-09-21 13:20:17 [SEVERE] Could not pass event EntityDeathEvent to LeechLife
Zeile 17618: 2012-09-21 13:20:00 [SEVERE] Could not pass event EntityDeathEvent to LeechLife
Zeile 17587: 2012-09-21 13:16:32 [SEVERE] Could not pass event EntityDeathEvent to LeechLife
Zeile 17557: 2012-09-21 13:16:32 [SEVERE] Could not pass event EntityDeathEvent to LeechLife
Zeile 17527: 2012-09-21 13:16:32 [SEVERE] Could not pass event EntityDeathEvent to LeechLife
Zeile 17497: 2012-09-21 13:16:31 [SEVERE] Could not pass event EntityDeathEvent to LeechLife
Zeile 17467: 2012-09-21 13:16:28 [SEVERE] Could not pass event EntityDeathEvent to LeechLife
Zeile 17437: 2012-09-21 13:16:27 [SEVERE] Could not pass event EntityDeathEvent to LeechLife
Zeile 17407: 2012-09-21 13:16:26 [SEVERE] Could not pass event EntityDeathEvent to LeechLife
Zeile 17377: 2012-09-21 13:16:26 [SEVERE] Could not pass event EntityDeathEvent to LeechLife
Zeile 17347: 2012-09-21 13:16:25 [SEVERE] Could not pass event EntityDeathEvent to LeechLife
Zeile 17317: 2012-09-21 13:16:24 [SEVERE] Could not pass event EntityDeathEvent to LeechLife
Zeile 17287: 2012-09-21 13:16:24 [SEVERE] Could not pass event EntityDeathEvent to LeechLife
Zeile 17257: 2012-09-21 13:16:20 [SEVERE] Could not pass event EntityDeathEvent to LeechLife
...
And so on.

Please provide any additional information below.

Maybe another Plugin interacts with the same Event. I used the Plugin in my "Mobarena" .  The only Plugin which interacts with the deaths of mobs too is "Jobs" but I bet it is an error with mobarena.