This is my error: 
at java.lang.Class.getDeclaredFields0(Native Method) 
	at java.lang.Class.privateGetDeclaredFields(Class.java:2575) 
	at java.lang.Class.getField0(Class.java:2967) 
	at java.lang.Class.getField(Class.java:1693) 
	at cpw.mods.fml.client.FMLClientHandler.detectOptifine(FMLClientHandler.java:277) 
	at cpw.mods.fml.client.FMLClientHandler.beginMinecraftLoading(FMLClientHandler.java:192) 
	at net.minecraft.client.Minecraft.func_71384_a(Minecraft.java:480) 
	at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:878) 
	at net.minecraft.client.main.Main.main(SourceFile:148) 
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) 
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) 
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) 
	at java.lang.reflect.Method.invoke(Method.java:483) 
	at net.minecraft.launchwrapper.Launch.launch(Launch.java:135) 
	at net.minecraft.launchwrapper.Launch.main(Launch.java:28) 
Caused by: java.lang.ClassNotFoundException: bxf 
	at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:191) 
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424) 
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357) 
	... 15 more 
Caused by: java.lang.NullPointerException 
	at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:182) 
	... 17 more 
  
Thanks for helping!