Once loaded mojang screen is fine press multiplayer/singleplayer ok press to join a server/ start my OFFLINE world it loads building terrain... back to menu screen then crash screen

---- Minecraft Crash Report ----
// Would you like a cupcake?

Time: 12/20/12 4:07 PM
Description: Ticking screen

java.lang.NoClassDefFoundError: la
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClassCond(ClassLoader.java:631)
at java.lang.ClassLoader.defineClass(ClassLoader.java:615)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:283)
at java.net.URLClassLoader.access$000(URLClassLoader.java:58)
at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
at java.lang.ClassLoader.loadClass(ClassLoader.java:295)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
at qx.<init>(SourceFile:51)
at azv.<init>(SourceFile:32)
at ayk.<init>(SourceFile:28)
at ayg.a(SourceFile:266)
at net.minecraft.client.Minecraft.a(SourceFile:1465)
at net.minecraft.client.Minecraft.a(SourceFile:1417)
at axz.a(SourceFile:132)
at dx.a(SourceFile:71)
at cg.b(SourceFile:349)
at axz.d(SourceFile:80)
at ayc.c(SourceFile:69)
at net.minecraft.client.Minecraft.l(SourceFile:1112)
at net.minecraft.client.Minecraft.J(SourceFile:582)
at net.minecraft.client.Minecraft.run(SourceFile:534)
at java.lang.Thread.run(Thread.java:680)
Caused by: java.lang.ClassNotFoundException: la
at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
... 28 more

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

– Head –
Stacktrace:
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClassCond(ClassLoader.java:631)
at java.lang.ClassLoader.defineClass(ClassLoader.java:615)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:283)
at java.net.URLClassLoader.access$000(URLClassLoader.java:58)
at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
at java.lang.ClassLoader.loadClass(ClassLoader.java:295)
at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
at qx.<init>(SourceFile:51)
at azv.<init>(SourceFile:32)
at ayk.<init>(SourceFile:28)
at ayg.a(SourceFile:266)
at net.minecraft.client.Minecraft.a(SourceFile:1465)
at net.minecraft.client.Minecraft.a(SourceFile:1417)
at axz.a(SourceFile:132)
at dx.a(SourceFile:71)
at cg.b(SourceFile:349)
at axz.d(SourceFile:80)
at ayc.c(SourceFile:69)

– Affected screen –
Details:
Screen name: ayc

– 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 ayh.a(SourceFile:290)
at net.minecraft.client.Minecraft.b(SourceFile:1870)
at net.minecraft.client.Minecraft.run(SourceFile:543)
at java.lang.Thread.run(Thread.java:680)

– System Details –
Details:
Minecraft Version: 1.4.5
Operating System: Mac OS X (x86_64) version 10.8.2
Java Version: 1.6.0_35, Apple Inc.
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Apple Inc.
Memory: 540391880 bytes (515 MB) / 581435392 bytes (554 MB) up to 1065025536 bytes (1015 MB)
JVM Flags: 3 total; -Xbootclasspath/a:/System/Library/PrivateFrameworks/JavaApplicationLauncher.framework/Resources/LauncherSupport.jar -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: Intel HD Graphics 3000 OpenGL Engine GL version 2.1 APPLE-8.0.61, Intel 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