Joining a server sometimes makes my Minecraft crash.

When I tried joining a Minecraft server, my Minecraft crashed for no reason. I didn't have any mods or mod loaders installed or anything. I spoke to my friends about this and they seemed to be getting the same problem. Here is the crash report:

Minecraft has crashed!      
      ----------------------      

Minecraft has stopped running because it encountered a problem; Ticking screen

A full error report has been saved to C:\Users\mcrazy\AppData\Roaming\.minecraft\crash-reports\crash-2013-05-23_16.54.34-client.txt - Please include a copy of that file (Not this screen!) if you report this crash to anyone; without it, they will not be able to help fix the crash :(



--- BEGIN ERROR REPORT b83c038e --------
Full report at:
C:\Users\mcrazy\AppData\Roaming\.minecraft\crash-reports\crash-2013-05-23_16.54.34-client.txt
Please show that file to Mojang, NOT just this screen!

Generated 5/23/13 4:54 PM

-- Head --
Stacktrace:
	at bdv.<clinit>(EntityClientPlayerMP.java:36)
	at bdr.a(SourceFile:269)
	at net.minecraft.client.Minecraft.a(SourceFile:1459)
	at net.minecraft.client.Minecraft.a(SourceFile:1411)
	at bdk.a(SourceFile:147)
	at dz.a(SourceFile:71)
	at ci.b(SourceFile:350)
	at bdk.d(SourceFile:95)
	at bdn.c(SourceFile:73)

-- Affected screen --
Details:
	Screen name: bdn

-- Affected level --
Details:
	Level name: MpServer
	All players: 0 total; []
	Chunk stats: MultiplayerChunkCache: 0
	Level seed: 0
	Level generator: ID 00 - default, ver 1. Features enabled: false
	Level generator options: 
	Level spawn location: World: (8,64,8), Chunk: (at 8,4,8 in 0,0; contains blocks 0,0,0 to 15,255,15), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
	Level time: 0 game time, 0 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: 0 total; []
	Retry entities: 0 total; []
Stacktrace:
	at bds.a(SourceFile:282)
	at net.minecraft.client.Minecraft.b(SourceFile:1881)
	at net.minecraft.client.Minecraft.run(SourceFile:535)
	at java.lang.Thread.run(Unknown Source)

-- System Details --
Details:
	Minecraft Version: 1.5.2
	Operating System: Windows 7 (amd64) version 6.1
	Java Version: 1.7.0_21, Oracle Corporation
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
	Memory: 422801952 bytes (403 MB) / 514523136 bytes (490 MB) up to 954466304 bytes (910 MB)
	JVM Flags: 2 total; -Xms512m -Xmx1024m
	AABB Pool Size: 10816 (605696 bytes; 0 MB) allocated, 10816 (605696 bytes; 0 MB) used
	Suspicious classes: No suspicious classes found.
	IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
	LWJGL: 2.4.2
	OpenGL: AMD Radeon HD 6670 GL version 4.2.12217 Compatibility Profile Context 12.104.0.0, ATI Technologies Inc.
	Is Modded: Very likely; Jar signature invalidated
	Type: Client (map_client.txt)
	Texture Pack: Default
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: 0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used

java.lang.NoClassDefFoundError: com/sijobe/spc/core/HookManager
	at bdv.<clinit>(EntityClientPlayerMP.java:36)
	at bdr.a(SourceFile:269)
	at net.minecraft.client.Minecraft.a(SourceFile:1459)
	at net.minecraft.client.Minecraft.a(SourceFile:1411)
	at bdk.a(SourceFile:147)
	at dz.a(SourceFile:71)
	at ci.b(SourceFile:350)
	at bdk.d(SourceFile:95)
	at bdn.c(SourceFile:73)
	at net.minecraft.client.Minecraft.l(SourceFile:1105)
	at net.minecraft.client.Minecraft.K(SourceFile:574)
	at net.minecraft.client.Minecraft.run(SourceFile:526)
	at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.ClassNotFoundException: com.sijobe.spc.core.HookManager
	at java.net.URLClassLoader$1.run(Unknown Source)
	at java.net.URLClassLoader$1.run(Unknown Source)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(Unknown Source)
	at java.lang.ClassLoader.loadClass(Unknown Source)
	at java.lang.ClassLoader.loadClass(Unknown Source)
	... 13 more
--- END ERROR REPORT c0e87ade ----------