The Mono.Posix test are periodically failing on the builders. The reason for this is that MonoTests.Mono.Unix.UnixSignalTest.TestWaitAny sometimes crashes or hangs, at least on OS X. I have observed this on the builders running Yosemite and on my laptop running El Capitan, with Mono versions in the 4.3.1-4.4 range. It can quickly be observed by running:
while (cd mcs/class/Mono.Posix MONO_PATH=../../../mcs/class/lib/net_4_x ../../../runtime/mono-wrapper --debug ../../class/lib/net_4_x/nunit-console.exe Mono.Posix_test_net_4_x.dll -noshadow -labels -exclude=NotOnMac,NotWorking,ValueAdd,CAS,InetAccess -run=MonoTests.Mono.Unix.UnixSignalTest.TestWaitAny); do :; done
It will usually happen after about thirty repetitions. On attaching LLDB, the stack where the crash is reported as occurring will always be found to be trashed; if the application is run in LLDB to start, the problem does not appear to reproduce at all.
The SIGSEGV seen here is addressed by PR #2264.