This is presumably a bug in Windows Media Video 9 Encoder in versions up to Windows 7 included (fixed in Windows 8.1 at the very least – wmvencod.dll 6.3.9600.17415). A IMediaObject::Discontinuity call destroys input the DMO it already holds: it reports success, and handles discontinuity correctly. It even drains output as it should, but if in the same time…