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I have never had any mods its all been working but now I cant play it anymore please help

---- Minecraft Crash Report ----
// Don't be sad. I'll do better next time, I promise!

Time: 12/13/12 3:33 PM
Description: Exception in world tick

java.lang.ArrayIndexOutOfBoundsException
at java.lang.System.arraycopy(Native Method)
at zs.a(SourceFile:820)
at axz.a(SourceFile:851)
at du.a(SourceFile:135)
at cg.b(SourceFile:349)
at axz.d(SourceFile:80)
at ayh.b(SourceFile:62)
at net.minecraft.client.Minecraft.l(SourceFile:1331)
at net.minecraft.client.Minecraft.J(SourceFile:582)
at net.minecraft.client.Minecraft.run(SourceFile:534)
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 java.lang.System.arraycopy(Native Method)
at zs.a(SourceFile:820)
at axz.a(SourceFile:851)
at du.a(SourceFile:135)
at cg.b(SourceFile:349)
at axz.d(SourceFile:80)

-- Affected level --
Details:
Level name: MpServer
All players: 1 total; [ayk['JackEdits'/3373, l='MpServer', x=117.50, y=75.62, z=-125.50]]
Chunk stats: MultiplayerChunkCache: 55
Level seed: 0
Level generator: ID 00 - default, ver 1. Features enabled: false
Level generator options:
Level spawn location: World: (117,74,-126), Chunk: (at 5,4,2 in 7,-8; contains blocks 112,0,-128 to 127,255,-113), Region: (0,-1; contains chunks 0,-32 to 31,-1, blocks 0,0,-512 to 511,255,-1)
Level time: 26529842 game time, 26529842 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: 14 total; [ayk['JackEdits'/3373, l='MpServer', x=117.50, y=75.62, z=-125.50], pe['Sheep'/1032, l='MpServer', x=71.63, y=74.00, z=-143.75], px['item.item.egg'/3340, l='MpServer', x=156.13, y=74.13, z=-127.50], px['item.item.egg'/3340, l='MpServer', x=156.13, y=74.13, z=-127.50], pe['Sheep'/1027, l='MpServer', x=70.47, y=74.00, z=-149.50], pe['Sheep'/1028, l='MpServer', x=71.03, y=74.00, z=-151.03], pe['Sheep'/1029, l='MpServer', x=79.03, y=74.00, z=-149.97], pe['Sheep'/1030, l='MpServer', x=78.84, y=74.00, z=-147.88], pe['Sheep'/1031, l='MpServer', x=70.47, y=74.00, z=-144.25], oy['Chicken'/1048, l='MpServer', x=161.41, y=74.00, z=-164.38], oy['Chicken'/1040, l='MpServer', x=132.53, y=74.00, z=-146.56], ph['Squid'/1042, l='MpServer', x=149.25, y=58.16, z=-170.19], oy['Chicken'/1045, l='MpServer', x=157.38, y=74.00, z=-127.56], oy['Chicken'/1047, l='MpServer', x=166.41, y=74.00, z=-164.41]]
Retry entities: 0 total; []
Stacktrace:
at ayh.a(SourceFile:290)
at net.minecraft.client.Minecraft.l(SourceFile:1338)
at net.minecraft.client.Minecraft.J(SourceFile:582)
at net.minecraft.client.Minecraft.run(SourceFile:534)
at java.lang.Thread.run(Unknown Source)

-- Affected level --
Details:
Level name: MpServer
All players: 1 total; [ayk['JackEdits'/3373, l='MpServer', x=117.50, y=75.62, z=-125.50]]
Chunk stats: MultiplayerChunkCache: 55
Level seed: 0
Level generator: ID 00 - default, ver 1. Features enabled: false
Level generator options:
Level spawn location: World: (117,74,-126), Chunk: (at 5,4,2 in 7,-8; contains blocks 112,0,-128 to 127,255,-113), Region: (0,-1; contains chunks 0,-32 to 31,-1, blocks 0,0,-512 to 511,255,-1)
Level time: 26529842 game time, 26529842 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: 14 total; [ayk['JackEdits'/3373, l='MpServer', x=117.50, y=75.62, z=-125.50], pe['Sheep'/1032, l='MpServer', x=71.63, y=74.00, z=-143.75], px['item.item.egg'/3340, l='MpServer', x=156.13, y=74.13, z=-127.50], px['item.item.egg'/3340, l='MpServer', x=156.13, y=74.13, z=-127.50], pe['Sheep'/1027, l='MpServer', x=70.47, y=74.00, z=-149.50], pe['Sheep'/1028, l='MpServer', x=71.03, y=74.00, z=-151.03], pe['Sheep'/1029, l='MpServer', x=79.03, y=74.00, z=-149.97], pe['Sheep'/1030, l='MpServer', x=78.84, y=74.00, z=-147.88], pe['Sheep'/1031, l='MpServer', x=70.47, y=74.00, z=-144.25], oy['Chicken'/1048, l='MpServer', x=161.41, y=74.00, z=-164.38], oy['Chicken'/1040, l='MpServer', x=132.53, y=74.00, z=-146.56], ph['Squid'/1042, l='MpServer', x=149.25, y=58.16, z=-170.19], oy['Chicken'/1045, l='MpServer', x=157.38, y=74.00, z=-127.56], oy['Chicken'/1047, l='MpServer', x=166.41, y=74.00, z=-164.41]]
Retry entities: 0 total; []

-- System Details --
Details:
Minecraft Version: 1.4.5
Operating System: Windows 7 (amd64) version 6.1
Java Version: 1.6.0_31, Sun Microsystems Inc.
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Sun Microsystems Inc.
Memory: 518121040 bytes (494 MB) / 648740864 bytes (618 MB) up to 954466304 bytes (910 MB)
JVM Flags: 2 total; -Xms512m -Xmx1024m
AABB Pool Size: 20035 (1121960 bytes; 1 MB) allocated, 178 (9968 bytes; 0 MB) used
Suspicious classes: No suspicious classes found.
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
LWJGL: 2.4.2
OpenGL: GeForce GT 520M/PCI/SSE2 GL version 4.2.0, NVIDIA Corporation
Is Modded: Probably not. Jar signature remains and client brand is untouched.
Type: Client (map_client.txt)
Texture Pack: Default
Profiler Position: N/A (disabled)
Vec3 Pool Size: 490 (27440 bytes; 0 MB) allocated, 111 (6216 bytes; 0 MB) used

I tend to get those randomly as well. Sometimes I get those due to not enough RAM, but usually that results in memory errors o.O

Its not ram issue I have heaps of ram spare