В случае с версией клиента чистой, прямо из архива такая же брешь, но лог другой:

"C:\Program Files (x86)\Java\jre6\bin\java.exe" -Xms560M -Xmx1024M -jar Minecraft.jar
Heap size: 989.0
Heap is big size
run complite
Exception in thread "Thread-2" java.lang.ClassFormatError: Truncated class file
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClassCond(Unknown Source)
at java.lang.ClassLoader.defineClass(Unknown Source)
at java.security.SecureClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.access$000(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)
at net.minecraft.client.MinecraftApplet.init(MinecraftApplet.java:33)
at net.minecraft.Launcher.replace(Launcher.java:166)
at net.minecraft.Launcher$1.run(Launcher.java:108)