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This could be the strangest crash report i've ever seen... does anyone have an idea what's happening and how to fix it?

---- Minecraft Crash Report ----

WARNING: coremods are present:
Contact their authors BEFORE contacting forge

// Don't do that.

Time: 1/11/17 9:30 AM
Description: Initializing game

java.lang.VerifyError: Bad return type
Exception Details:
Location:
Config.getDefiningResourcePack(Lnet/minecraft/network/datasync/EntityDataManager;)Lnet/minecraft/client/renderer/entity/layers/LayerDeadmau5Head; @48: areturn
Reason:
Type 'net/minecraft/client/renderer/entity/RenderWitherSkeleton' (current frame, stack[0]) is not assignable to 'net/minecraft/client/renderer/entity/layers/LayerDeadmau5Head' (from method signature)
Current Frame:
bci: @48
flags: { }
locals: { 'net/minecraft/network/datasync/EntityDataManager', '[Lnet/minecraft/client/renderer/entity/layers/LayerDeadmau5Head;' }
stack: { 'net/minecraft/client/renderer/entity/RenderWitherSkeleton' }
Bytecode:
0x0000000: b803 4b4c 2bbe 0464 3d1c 9b00 192b 1c32
0x0000010: 4e2d 2ab9 0375 0200 9900 052d b084 02ff
0x0000020: a7ff e9b8 034f 2ab6 0376 9900 07b8 034f
0x0000030: b001 b0
Stackmap Table:
append_frame(@9,Object[#830],Integer)
same_frame(@29)
chop_frame(@35,1)
same_frame(@49)

at java.lang.Class.getDeclaredFields0(Native Method)
at java.lang.Class.privateGetDeclaredFields(Class.java:2583)
at java.lang.Class.getField0(Class.java:2975)
at java.lang.Class.getField(Class.java:1701)
at net.minecraftforge.fml.client.FMLClientHandler.detectOptifine(FMLClientHandler.java:300)
at net.minecraftforge.fml.client.FMLClientHandler.beginMinecraftLoading(FMLClientHandler.java:211)
at net.minecraft.client.Minecraft.func_71384_a(Minecraft.java:440)
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:352)
at net.minecraft.client.main.Main.main(SourceFile:124)
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:498)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:135)
at net.minecraft.launchwrapper.Launch.main(Launch.java:28)

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

-- Head --
Thread: Client thread
Stacktrace:
at java.lang.Class.getDeclaredFields0(Native Method)
at java.lang.Class.privateGetDeclaredFields(Class.java:2583)
at java.lang.Class.getField0(Class.java:2975)
at java.lang.Class.getField(Class.java:1701)
at net.minecraftforge.fml.client.FMLClientHandler.detectOptifine(FMLClientHandler.java:300)
at net.minecraftforge.fml.client.FMLClientHandler.beginMinecraftLoading(FMLClientHandler.java:211)
at net.minecraft.client.Minecraft.func_71384_a(Minecraft.java:440)

-- Initialization --
Details:
Stacktrace:
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:352)
at net.minecraft.client.main.Main.main(SourceFile:124)
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:498)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:135)
at net.minecraft.launchwrapper.Launch.main(Launch.java:28)

-- System Details --
Details:
Minecraft Version: 1.11.2
Operating System: Mac OS X (x86_64) version 10.11.5
Java Version: 1.8.0_74, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 90597208 bytes (86 MB) / 255778816 bytes (243 MB) up to 1060372480 bytes (1011 MB)
JVM Flags: 5 total; -Xmx1G -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
FML:
Loaded coremods (and transformers):
Launched Version: 1.11.2-forge1.11.2-13.20.0.2206
LWJGL: 2.9.2
OpenGL: Intel HD Graphics 5000 OpenGL Engine GL version 2.1 INTEL-10.14.66, Intel Inc.
GL Caps: Using GL 1.3 multitexturing.
Using GL 1.3 texture combiners.
Using framebuffer objects because ARB_framebuffer_object is supported and separate blending is supported.
Shaders are available because OpenGL 2.1 is supported.
VBOs are available because OpenGL 1.5 is supported.

Using VBOs: Yes
Is Modded: Definitely; Client brand changed to 'fml,forge'
Type: Client (map_client.txt)
Resource Packs:
Current Language: ~~ERROR~~ NullPointerException: null
Profiler Position: N/A (disabled)
CPU: 4x Intel(R) Core(TM) i5-4260U CPU @ 1.40GHz

Post a DxDiag: http://hopper.minecraft.net/help/pixel-format-not-accelerated/support/#dxdiag

Also does it crash when you open a world or when you launch minecraft?