Bukkit

What steps will reproduce the problem?
I dont know. It's happen randomly.

What is the expected output? What do you see instead?
i tryed to turn off other plugins, but this no effect.

What version of the product are you using?
1.3

Do you have an error log of what happened?
Yes
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[22:31:57] [pool-3-thread-4/WARN]: Exception in thread "pool-3-thread-4"
[22:31:57] [pool-3-thread-4/WARN]: org.apache.commons.lang.UnhandledException: Plugin PvPTimer v1.3 generated an exception while executing task 24
    at org.bukkit.craftbukkit.v1_7_R1.scheduler.CraftAsyncTask.run(CraftAsyncTask.java:56)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
    at java.lang.Thread.run(Thread.java:722)
Caused by: java.util.ConcurrentModificationException
    at java.util.HashMap$HashIterator.nextEntry(HashMap.java:806)
    at java.util.HashMap$KeyIterator.next(HashMap.java:841)
    at lemon42.PvPTimer.CheckTask.run(CheckTask.java:13)
    at org.bukkit.craftbukkit.v1_7_R1.scheduler.CraftTask.run(CraftTask.java:53)
    at org.bukkit.craftbukkit.v1_7_R1.scheduler.CraftAsyncTask.run(CraftAsyncTask.java:53)
    ... 3 more
-----------------------------

Please provide any additional information below.
It seems like this issue: http://dev.bukkit.org/bukkit-plugins/pvptimer/tickets/16-keep-getting-this-error-stack/
But i am not sure.
Sorry for my Eng..
Thanks,
pAxs1s