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I really don't know what kind of title to give this. Whenever I try loading a world/creating a new world, a shutting server down message appears, and this error status shows up

---- Minecraft Crash Report ----
// Oh - I know what I did wrong!

Time: 4/20/13 6:19 PM
Description: Unexpected error

java.lang.ClassCastException: net.minecraft.client.entity.EntityClientPlayerMP cannot be cast to cpw.mods.fml.common.network.Player
at cpw.mods.fml.common.network.NetworkRegistry.handleCustomPacket(NetworkRegistry.java:237)
at cpw.mods.fml.common.network.FMLNetworkHandler.handlePacket250Packet(FMLNetworkHandler.java:83)
at net.minecraft.client.multiplayer.NetClientHandler.func_72501_a(NetClientHandler.java:1571)
at net.minecraft.network.packet.Packet250CustomPayload.func_73279_a(SourceFile:59)
at net.minecraft.network.MemoryConnection.func_74428_b(MemoryConnection.java:89)
at net.minecraft.client.Minecraft.func_71407_l(Minecraft.java:1895)
at net.minecraft.client.Minecraft.func_71411_J(Minecraft.java:831)
at net.minecraft.client.Minecraft.run(Minecraft.java:756)
at java.lang.Thread.run(Unknown Source)


A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Stacktrace:
at cpw.mods.fml.common.network.NetworkRegistry.handleCustomPacket(NetworkRegistry.java:237)
at cpw.mods.fml.common.network.FMLNetworkHandler.handlePacket250Packet(FMLNetworkHandler.java:83)
at net.minecraft.client.multiplayer.NetClientHandler.func_72501_a(NetClientHandler.java:1571)
at net.minecraft.network.packet.Packet250CustomPayload.func_73279_a(SourceFile:59)
at net.minecraft.network.MemoryConnection.func_74428_b(MemoryConnection.java:89)

-- Affected level --
Details:
Level name: MpServer
All players: 1 total; [EntityClientPlayerMP['1cheesepuff'/23, l='MpServer', x=-261.50, y=64.62, z=223.50]]
Chunk stats: MultiplayerChunkCache: 0
Level seed: 0
Level generator: ID 00 - default, ver 1. Features enabled: false
Level generator options:
Level spawn location: World: (-252,64,232), Chunk: (at 4,4,8 in -16,14; contains blocks -256,0,224 to -241,255,239), Region: (-1,0; contains chunks -32,0 to -1,31, blocks -512,0,0 to -1,255,511)
Level time: 1 game time, 1 day time
Level dimension: 0
Level storage version: 0x00000 - Unknown?
Level weather: Rain time: 0 (now: false), thunder time: 0 (now: false)
Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Forced entities: 1 total; [EntityClientPlayerMP['1cheesepuff'/23, l='MpServer', x=-261.50, y=64.62, z=223.50]]
Retry entities: 0 total; []
Stacktrace:
at net.minecraft.client.multiplayer.WorldClient.func_72914_a(WorldClient.java:441)
at net.minecraft.client.Minecraft.func_71396_d(Minecraft.java:2405)
at net.minecraft.client.Minecraft.run(Minecraft.java:779)
at java.lang.Thread.run(Unknown Source)

-- System Details --
Details:
Minecraft Version: 1.5.1
Operating System: Windows 7 (x86) version 6.1
Java Version: 1.7.0_17, Oracle Corporation
Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Oracle Corporation
Memory: 383769024 bytes (365 MB) / 519110656 bytes (495 MB) up to 1037959168 bytes (989 MB)
JVM Flags: 2 total; -Xms512m -Xmx1024m
AABB Pool Size: 10817 (605752 bytes; 0 MB) allocated, 10817 (605752 bytes; 0 MB) used
Suspicious classes: FML and Forge are installed
IntCache: cache: 0, tcache: 0, allocated: 3, tallocated: 63
FML: MCP v7.44 FML v5.1.8.611 Minecraft Forge 7.7.1.611 4 mods loaded, 4 mods active
mcp [Minecraft Coder Pack] (minecraft.jar) Unloaded->Constructed->Pre-initialized->Initialized->Post-initialized->Available->Available->Available->Available
FML [Forge Mod Loader] (coremods) Unloaded->Constructed->Pre-initialized->Initialized->Post-initialized->Available->Available->Available->Available
Forge [Minecraft Forge] (coremods) Unloaded->Constructed->Pre-initialized->Initialized->Post-initialized->Available->Available->Available->Available
Backpack [Backpack] (backpack-1.7.8-1.5.1.zip) Unloaded->Constructed->Pre-initialized->Initialized->Post-initialized->Available->Available->Available->Available
LWJGL: 2.4.2
OpenGL: GeForce GT 530/PCIe/SSE2 GL version 4.3.0, NVIDIA Corporation
Is Modded: Definitely; Cli