Nikolay Sivov (@nsivov) commented about dlls/mf/tests/mf.c:
return;- }
 - /* Set presentation clock. */
 - hr = MFCreateSystemTimeSource(&time_source);
 - ok(hr == S_OK, "Unexpected hr %#lx.\n", hr);
 - hr = MFCreatePresentationClock(&clock);
 - ok(hr == S_OK, "Unexpected hr %#lx.\n", hr);
 - hr = IMFPresentationClock_SetTimeSource(clock, time_source);
 - ok(hr == S_OK, "Unexpected hr %#lx.\n", hr);
 - hr = IMFMediaSink_SetPresentationClock(media_sink, clock);
 - ok(hr == S_OK, "Unexpected hr %#lx.\n", hr);
 - /* Stream event handler. */
 - hr = IMFMediaSink_GetStreamSinkById(media_sink, 1, &stream_sink);
 - ok(hr == S_OK, "Unexpected hr %#lx.\n", hr);
 
Why is that at index 1? What's at 0?