We've seen instances reported of base_unittests flaking on Fuchsia with output of the form:

---
[00040.815] <== fatal exception: process /pkg/base_unittests__exec[202069] thread TaskSchedulerSingleThreadBackgr[203146]
[00040.815] <== undefined instruction, PC at 0x5f363b74b87c
[00040.816]  CS:                   0 RIP:     0x5f363b74b87c EFL:            0x10202 CR2:                  0
[00040.816]  RAX:                0xa RBX:     0x5f363b804028 RCX:     0x63ba31800828 RDX:     0x5f363b8040b4
[00040.816]  RSI:      0x9a7512426d8 RDI:     0x5f363b804028 RBP:      0x9a751242c40 RSP:      0x9a751242878
[00040.816]   R8:                  0  R9:                  0 R10:                  0 R11:              0x246
[00040.816]  R12:      0x9a751242b28 R13:      0x9a751242a08 R14:      0x9a751242da8 R15:                  0
[00040.816]  errc:                 0
[00040.816] bottom of user stack:
[00040.816] 0x000009a751242878: 3fad0690 00002d74 00000000 00000000 |...?t-..........|
[00040.816] 0x000009a751242888: 51242da8 000009a7 51242c40 000009a7 |.-$Q....@,$Q....|
[00040.816] 0x000009a751242898: 00000070 00000000 00000318 00000000 |p...............|
[00040.816] 0x000009a7512428a8: 1c9915d5 00000000 00000043 00000000 |........C.......|
[00040.816] 0x000009a7512428b8: 3f07eb22 00002d74 51242b28 000009a7 |"..?t-..(+$Q....|
[00040.816] 0x000009a7512428c8: 51242a08 000009a7 5b97af67 8202d3b6 |.*$Q....g..[....|
[00040.816] 0x000009a7512428d8: 51242c40 000009a7 51242cb0 000009a7 |@,$Q.....,$Q....|
[00040.816] 0x000009a7512428e8: 51242c48 000009a7 3f07eb3b 00002d74 |H,$Q....;..?t-..|
[00040.816] 0x000009a7512428f8: 3f07eb22 00002d74 51242c40 000009a7 |"..?t-..@,$Q....|
[00040.816] 0x000009a751242908: 3f375169 00002d74 3fda5358 00002d74 |iQ7?t-..XS.?t-..|
[00040.816] 0x000009a751242918: 3fa9fb01 00002d74 51242c40 000009a7 |...?t-..@,$Q....|
[00040.816] 0x000009a751242928: 00000043 00000000 00000008 00000030 |C...........0...|
[00040.816] 0x000009a751242938: 51242960 000009a7 51242880 000009a7 |`)$Q.....($Q....|
[00040.816] 0x000009a751242948: 51242a08 000009a7 9b7fa7f8 000053e4 |.*$Q.........S..|
[00040.816] 0x000009a751242958: 3fad1587 00002d74 3fd7a590 00002d74 |...?t-.....?t-..|
[00040.816] 0x000009a751242968: 3fa9fe8d 00002d74 00000000 00000000 |...?t-..........|
[00040.816] arch: x86_64
[00040.816] dso: id=67a9a5b689915265e28cdaf84f84dfc853b73995 base=0x780aa3361000 name=libfdio.so
[00040.816] dso: id=c2f4882d7a78c26114ded2b85bb8b07703e258de base=0x63ba317f9000 name=<vDSO>
[00040.816] dso: id=55585d12cd93db9a1105ccb59a1155ff8982147e base=0x5f363b735000 name=libc.so
[00040.816] dso: id=b8426d36179cc65d646097ad3ec03bad4316420b base=0x4549aaaee000 name=libtrace-engine.so
[00040.816] dso: id=3f13bd7ae93ac61d base=0x2d743ef61000 name=app:/pkg/base_unittests__exec
[00040.816] dso: id=5e3c55aea09e8943ae76d3603cc8c0b1ba27bfb7 base=0xc6cfdba4000 name=libasync-default.so
[00040.816] dso: id=11ea1b2cadd51f86 base=0x3d669bfe000 name=libsvc.so
[00040.816] {{{reset}}}
[00040.816] {{{module:0x34646162:<VMO#202136=libsvc.so>:elf:11ea1b2cadd51f86}}}
[00040.816] {{{mmap:0x3d669bfe000:0x6000:load:0x34646162:r:0}}}
[00040.816] {{{mmap:0x3d669c04000:0x11000:load:0x34646162:rx:0x6000}}}
[00040.816] {{{mmap:0x3d669c15000:0x4000:load:0x34646162:rw:0x17000}}}
[00040.816] {{{module:0x34646163:<VMO#202120=libasync-default.so:elf:5e3c55aea09e8943ae76d3603cc8c0b1ba27bfb7}}}
[00040.816] {{{mmap:0xc6cfdba4000:0x1000:load:0x34646163:rx:0}}}
[00040.816] {{{mmap:0xc6cfdba5000:0x1000:load:0x34646163:rw:0x1000}}}
[00040.816] {{{module:0x34646164:<VMO#202128=data:libasync-defau:elf:5e3c55aea09e8943ae76d3603cc8c0b1ba27bfb7}}}
[00040.816] {{{mmap:0xc6cfdba5000:0x1000:load:0x34646164:rx:0}}}
[00040.816] {{{mmap:0xc6cfdba6000:0x1000:load:0x34646164:rw:0x1000}}}
[00040.816] {{{module:0x34646165:<VMO#202056=/pkg/base_unittests:elf:3f13bd7ae93ac61d}}}
[00040.816] {{{mmap:0x2d743ef61000:0x413000:load:0x34646165:r:0}}}
[00040.816] {{{mmap:0x2d743f374000:0x9a3000:load:0x34646165:rx:0x413000}}}
[00040.816] {{{mmap:0x2d743fd17000:0x96000:load:0x34646165:rw:0xdb6000}}}
[00040.816] {{{module:0x34646166:<VMO#202147=libtrace-engine.so>:elf:b8426d36179cc65d646097ad3ec03bad4316420b}}}
[00040.816] {{{mmap:0x4549aaaee000:0x9000:load:0x34646166:rx:0}}}
[00040.816] {{{mmap:0x4549aaaf7000:0x2000:load:0x34646166:rw:0x9000}}}
[00040.816] {{{module:0x34646167:<VMO#202073=ld.so.1>:elf:55585d12cd93db9a1105ccb59a1155ff8982147e}}}
[00040.816] {{{mmap:0x5f363b735000:0xcb000:load:0x34646167:rx:0}}}
[00040.816] {{{mmap:0x5f363b801000:0x6000:load:0x34646167:rw:0xcc000}}}
[00040.816] {{{module:0x34646168:<VMO#1033=vdso/full>:elf:c2f4882d7a78c26114ded2b85bb8b07703e258de}}}
[00040.816] {{{mmap:0x63ba317f9000:0x7000:load:0x34646168:r:0}}}
[00040.816] {{{mmap:0x63ba31800000:0x1000:load:0x34646168:rx:0x7000}}}
[00040.816] {{{module:0x34646169:<VMO#202094=libfdio.so>:elf:67a9a5b689915265e28cdaf84f84dfc853b73995}}}
[00040.816] {{{mmap:0x780aa3361000:0x22000:load:0x34646169:rx:0}}}
[00040.816] {{{mmap:0x780aa3383000:0x5000:load:0x34646169:rw:0x22000}}}
#01: pc 0x5f363b74b87c sp 0x9a751242878 (libc.so,0x1687c)
#02: __cxa_bad_typeid at cxa_aux_runtime.cpp:?
#03: __cxxabiv1::__aligned_malloc_with_fallback(unsigned long) at fallback_malloc.cpp:?
#04: testing::internal::UntypedFunctionMockerBase::UntypedInvokeWith(void*) at gmock-spec-builders.cc:?
#05: testing::internal::FunctionMockerBase<void (int)>::InvokeWith(std::__1::tuple<int>&&) at bind_unittest.cc:?
#06: testing::internal::FunctionMocker<void ()>::Invoke() at bind_unittest.cc:?
#07: base::internal::SchedulerWorker::RunWorker() at scheduler_worker.cc:?
#08: base::internal::SchedulerWorker::RunDedicatedWorker() at scheduler_worker.cc:?
#09: base::(anonymous namespace)::ThreadFunc(void*) at platform_thread_posix.cc:?
#10: pc 0x5f363b74c636 sp 0x9a751242fd0 (libc.so,0x17636)
#11: pc 0x5f363b7bbad1 sp 0x9a751242fe0 (libc.so,0x86ad1)
#12: pc 0 sp 0x9a751243000
[00040.818] {{{bt:1:0x5f363b74b87c}}}
[00040.818] {{{bt:2:0x2d743fad0690}}}
[00040.818] {{{bt:3:0x2d743fad1587}}}
[00040.818] {{{bt:4:0x2d743fa9fe8d}}}
[00040.818] {{{bt:5:0x2d743f38963c}}}
[00040.818] {{{bt:6:0x2d743f38961c}}}
[00040.819] {{{bt:7:0x2d743fb6b84a}}}
[00040.819] {{{bt:8:0x2d743fb6b70f}}}
[00040.819] {{{bt:9:0x2d743fbccb02}}}
[00040.819] {{{bt:10:0x5f363b74c636}}}
[00040.819] {{{bt:11:0x5f363b7bbad1}}}
[00040.819] {{{bt:12:0}}}
[00041.041] [204026:204038:20181206,004403.376398:WARNING process_reader_fuchsia.cc:283] zx_thread_read_state(ZX_THREAD_STATE_GENERAL_REGS): ZX_ERR_BAD_STATE (-20)
[00041.041] [204026:204038:20181206,004403.376443:WARNING process_reader_fuchsia.cc:295] zx_thread_read_state(ZX_THREAD_STATE_VECTOR_REGS): ZX_ERR_BAD_STATE (-20)
[00041.095] [204026:204038:20181206,004403.430811:ERROR file_io_posix.cc:148] open /data/crashes/settings.dat.__lock__: File exists (17)
[00041.242] pkgsvr: pkgfs:unsupported(/packages/base_unittests/0): dir unlink "base_unittests__exec.log"
[00041.274] pkgsvr: pkgfs:unsupported(/packages/base_unittests/0): dir unlink "base_unittests__exec.log"
[3851/4037] OneTaskSchedulerImplTestParams/TaskSchedulerImplTest.IdentifiableStacks/29 (5 ms)
[3852/4037] OneTaskSchedulerImplTestParams/TaskSchedulerImplTest.IdentifiableStacks/30 (4 ms)
[3853/4037] OneTaskSchedulerImplTestParams/TaskSchedulerImplTest.IdentifiableStacks/31 (2 ms)
[3854/4037] OneTaskSchedulerImplTestParams/TaskSchedulerImplTest.IdentifiableStacks/32 (5 ms)
[3855/4037] OneTaskSchedulerImplTestParams/TaskSchedulerImplTest.IdentifiableStacks/33 (15 ms)
[3856/4037] OneTaskSchedulerImplTestParams/TaskSchedulerImplTest.IdentifiableStacks/34 (7 ms)
[3857/4037] OneTaskSchedulerImplTestParams/TaskSchedulerImplTest.IdentifiableStacks/35 (5 ms)
[3858/4037] OneTaskSchedulerImplTestParams/TaskSchedulerImplTest.SchedulerWorkerObserver/0 (8 ms)
[ RUN      ] OneTaskSchedulerImplTestParams/TaskSchedulerImplTest.SchedulerWorkerObserver/1
../../base/task/task_scheduler/task_scheduler_impl_unittest.cc:901: Failure
Actual function call count doesn't match EXPECT_CALL(observer, OnSchedulerWorkerMainExit())...
         Expected: to be called 10 times
           Actual: called 9 times - unsatisfied and active
Stack trace:
#00: testing::internal::UnitTestImpl::CurrentOsStackTraceExceptTop(int) at gtest.cc:?
#01: testing::internal::AssertHelper::operator=(testing::Message const&) const at gtest.cc:?
#02: testing::internal::GoogleTestFailureReporter::ReportFailure(testing::internal::FailureReporterInterface::FailureType, char const*, int, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&) at gmock-internal-utils.cc:?
#03: testing::internal::UntypedFunctionMockerBase::VerifyAndClearExpectationsLocked() at gmock-spec-builders.cc:?
#04: testing::internal::FunctionMockerBase<void ()>::~FunctionMockerBase() at bind_unittest.cc:?
#05: base::internal::test::MockSchedulerWorkerObserver::~MockSchedulerWorkerObserver() at test_utils.cc:?
#06: base::internal::TaskSchedulerImplTest_SchedulerWorkerObserver_Test::TestBody() at task_scheduler_impl_unittest.cc:?
[00041.301] pkgsvr: pkgfs:unsupported(/packages/base_unittests/0): dir unlink "base_unittests__exec.log"

Pure virtual function called!
---

It looks like the test is tearing down a mock object while there are still tasks pending which will touch it.