client.annotation-concurrency.bb-truncate* tests fail flakily

Running the test suite on Win7 with VS2010 and Ninja I see these 2 tests fail on occasion:

debug-internal-64: 150 tests passed, **** 2 tests failed: ****
        code_api|client.annotation-concurrency.bb-truncate-2 
        code_api|client.annotation-concurrency.bb-truncate-1 

My test output seems to be missing a lot of things:

135,259d134
<       <In context 0 at event 1, the mode changed from 0 to 1>
<       <In context 0 at event 2, the mode changed from 1 to 0>
<       <In context 0 at event 3, the mode changed from 0 to 1>
<       <In context 0 at event 4, the mode changed from 1 to 0>
<       <In context 0 at event 5, the mode changed from 0 to 1>
<       <In context 0 at event 6, the mode changed from 1 to 0>
<       <In context 0 at event 7, the mode changed from 0 to 1>
<       <In context 0 at event 8, the mode changed from 1 to 0>
<       <In context 0 at event 9, the mode changed from 0 to 1>
<       <In context 0 at event 10, the mode changed from 1 to 0>
<       <In context 0 at event 11, the mode changed from 0 to 1>
<       <In context 0 at event 12, the mode changed from 1 to 0>
<       <In context 0 at event 13, the mode changed from 0 to 1>
<many more like this>

In another suite run, just the -1 test failed.

When run manually I cannot reproduce:

% for ((i=0; i<10; i++)); do ctest -R concurrency.bb-truncate-1; done

=> all 10 pass