http://webkit-test-results.appspot.com/dashboards/flakiness_dashboard.html#showAllRuns=true&tests=compositing%2Freflections%2Fmasked-reflection-on-composited.html
This is a com.apple.coremedia.videomediaconverter thread crash that happens on may tests randomly. Eric, do you know if this is covered by a Radar? Thread 28 Crashed:: com.apple.coremedia.videomediaconverter 0 com.apple.AppKit 0x00007fff9015b060 -[NSApplication _crashOnException:] + 109 1 com.apple.AppKit 0x00007fff9015b4eb -[NSApplication reportException:] + 140 2 com.apple.CoreFoundation 0x00007fff8e830b1e __handleUncaughtException + 718 3 libobjc.A.dylib 0x00007fff916f17cd _objc_terminate() + 94 4 libc++abi.dylib 0x00007fff8d9e10a1 std::__terminate(void (*)()) + 8 5 libc++abi.dylib 0x00007fff8d9e0b30 __cxa_throw + 121 6 libobjc.A.dylib 0x00007fff916ed898 objc_exception_throw + 341 7 com.apple.CoreFoundation 0x00007fff8e6fadf4 -[__NSArrayM objectAtIndex:] + 260 8 com.apple.MediaToolbox 0x00007fff86360a0d 0x7fff862a1000 + 784909 9 com.apple.CoreMedia 0x00007fff90d5ec8f fncHandleWeakNotification + 67
<rdar://problem/19745369>
This should be fixed, see rdar://19745369