Minecraft crash

---- Minecraft Crash Report ----
// Ouch. That hurt

Time: 6/08/14 12:27
Description: Exception in server tick loop

java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextEntry(Unknown Source)
at java.util.HashMap$KeyIterator.next(Unknown Source)
at qi.a(SourceFile:163)
at net.minecraft.server.MinecraftServer.y(SourceFile:614)
at net.minecraft.server.MinecraftServer.x(SourceFile:530)
at cxc.x(SourceFile:155)
at net.minecraft.server.MinecraftServer.run(SourceFile:446)
at java.lang.Thread.run(Unknown Source)

this is the crash report, many times minecraft crash in little time period