Happens in webgl2_conformance_tests on Intel and ATI GPUs, for example here:
https://build.chromium.org/p/chromium.gpu.fyi/builders/Mac%20GPU%20ASAN%20Release/builds/1675

Error log is:

Init testcase: fence_sync.client_wait_sync_finish
Case iteration 1 / 5
Primitives drawn.
Sync object created
glClientWaitSync return value:
gl.TIMEOUT_EXPIRED
Expected glClientWaitSync to return gl.ALREADY_SIGNALED.
Test result: Failed!
glClientWaitSync return value:
gl.TIMEOUT_EXPIRED
Expected glClientWaitSync to return gl.ALREADY_SIGNALED.
Test result: Failed!
glClientWaitSync return value:
gl.ALREADY_SIGNALED
Sync object deleted.
Test result: Passed!
Sync objects created and deleted successfully.
Case iteration 2 / 5
Primitives drawn.
Sync object created
glClientWaitSync return value:
gl.TIMEOUT_EXPIRED
Expected glClientWaitSync to return gl.ALREADY_SIGNALED.
Test result: Failed!
glClientWaitSync return value:
gl.ALREADY_SIGNALED
Sync object deleted.
Test result: Passed!
Sync objects created and deleted successfully.
Case iteration 3 / 5
Primitives drawn.
Sync object created
glClientWaitSync return value:
gl.TIMEOUT_EXPIRED
Expected glClientWaitSync to return gl.ALREADY_SIGNALED.
Test result: Failed!
glClientWaitSync return value:
gl.TIMEOUT_EXPIRED
Expected glClientWaitSync to return gl.ALREADY_SIGNALED.
Test result: Failed!
glClientWaitSync return value:
gl.ALREADY_SIGNALED
Sync object deleted.
Test result: Passed!
Sync objects created and deleted successfully.
Case iteration 4 / 5
Primitives drawn.
Sync object created
glClientWaitSync return value:
gl.TIMEOUT_EXPIRED
Expected glClientWaitSync to return gl.ALREADY_SIGNALED.
Test result: Failed!
glClientWaitSync return value:
gl.TIMEOUT_EXPIRED
Expected glClientWaitSync to return gl.ALREADY_SIGNALED.
Test result: Failed!
glClientWaitSync return value:
gl.ALREADY_SIGNALED
Sync object deleted.
Test result: Passed!
Sync objects created and deleted successfully.
Case iteration 5 / 5
Primitives drawn.
Sync object created
glClientWaitSync return value:
gl.TIMEOUT_EXPIRED
Expected glClientWaitSync to return gl.ALREADY_SIGNALED.
Test result: Failed!
FAIL Fail
Sync object deleted.

The history where it started is lost, it was already happening here, for example:
https://build.chromium.org/p/chromium.gpu.fyi/builders/Mac%20GPU%20ASAN%20Release/builds/878

This also happens on non ASAN bots, but I guess less frequently:
 issue 687374 ,  issue 676848 

Going to suppress as Fail, as I expect Flaky will just fail all retries.