Help what does this mean? (Signs crash server)

Help what does this mean?

Code:

03:55 PM [SEVERE] java.lang.NoSuchMethodError: net.minecraft.server.NBTTagCompound.a(Ljava/lang/String;I)V
03:55 PM [SEVERE] at com.bergerkiller.bukkit.sl.TileEntityVirtualSign.b(TileEntityVirtualSign.java:34)
03:55 PM [SEVERE] at net.minecraft.server.ChunkLoader.a(SourceFile:135)
03:55 PM [SEVERE] at net.minecraft.server.ChunkRegionLoader.a(SourceFile:84)
03:55 PM [SEVERE] at net.minecraft.server.ChunkProviderServer.saveChunk(ChunkProviderServer.java:152)
03:55 PM [SEVERE] at net.minecraft.server.ChunkProviderServer.saveChunks(ChunkProviderServer.java:201)
03:55 PM [SEVERE] at net.minecraft.server.World.save(World.java:269)
03:55 PM [SEVERE] at net.minecraft.server.World.doTick(World.java:1735)
03:55 PM [SEVERE] at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:514)
03:55 PM [SEVERE] at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:425)
03:55 PM [SEVERE] at net.minecraft.server.ThreadServerApplication.run(SourceFile:457)
03:55 PM [SEVERE] Unexpected exception

 

When does this error happen? Do you have any plugins that have anything to do with how the world saves?