How to reproduce:
1. Open the attached "CrashMetal" project and "ShowcaseSceneContactShadows" scene
2. Enable "Main Camera" GameObject

Result - Crash occurs on:
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0 libsystem_kernel.dylib 0x00007fff9ca68d42 __pthread_kill + 10
1 libsystem_c.dylib 0x00007fff9c9ce420 abort + 129
2 libsystem_c.dylib 0x00007fff9c995893 basename_r + 0
3 Metal 0x00007fff8aa8de0b MTLReportFailure + 822
4 Metal 0x00007fff8aa6a73b -[MTLTextureDescriptorInternal validateWithDevice:] + 946
5 GeForceMTLDriver 0x00007fff839314c3 GeForceMTLDriver + 947395
6 GeForceMTLDriver 0x00007fff838e1060 GeForceMTLDriver + 618592
7 Unity 0x0000000102cc7961 _ZL22InitRenderSurfaceMetalR18RenderSurfaceMetal14MTLPixelFormat + 1137
8 Unity 0x0000000102cc746a _ZN5metal24CreateColorRenderSurfaceEP23ColorRenderSurfaceMetal19RenderTextureFormat + 58
9 Unity 0x0000000102cbadbe _ZN14GfxDeviceMetal32CreateColorRenderSurfacePlatformEP17RenderSurfaceBase19RenderTextureFormat + 14
10 Unity 0x0000000101a67740 _ZN15GfxDeviceWorker10RunCommandER20ThreadedStreamBuffer + 1984
11 Unity 0x0000000102cbdbdc GfxDeviceWorkerAutoreleasePoolProxy + 60
12 Unity 0x0000000101a70a54 _ZN15GfxDeviceWorker3RunEv + 116
13 Unity 0x0000000101a66e71 _ZN15GfxDeviceWorker18RunGfxDeviceWorkerEPv + 81
14 Unity 0x00000001022430fa _ZN6Thread16RunThreadWrapperEPv + 90
15 libsystem_pthread.dylib 0x00007fff9cb5393b _pthread_body + 180
16 libsystem_pthread.dylib 0x00007fff9cb53887 _pthread_body + 0
17 libsystem_pthread.dylib 0x00007fff9cb5308d thread_start + 13
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Reproducible with - 5.6.0a5, 5.6.3p3, 2017.1.0b1
Not reproducible with - 2017.1.0b2 (fixed in this version) and later

Fixed in - 5.6.6f2

Note: Crash seems to be caused by this line of code "computeShadowsCB.GetTemporaryRT(cShadow, -1,-1,0, FitlerMode.Billinear, RenderTextureFormat.R8);" in the NGSS_ContactShadows.cs file. Commenting it prevents the crash