Incorrect serialno setup in recent Tremor if file is not seekable.
In r16259 of vorbisfile.c in Tremor, the line 944 doesn't look correct. From what I can tell, vf->current_serialno is not yet set, so vf->serialnos is incorrect when _ov_open1 returns. If the file is seekable, this will be updated to the correct value by _open_seekable2. If the file isn't seekable, no correction is made.