when trying to import the following files
http://urchn.org/pitivi/samples/fly.m4v
and
http://urchn.org/pitivi/samples/hop.m4v

I get the following errors, and the preview pane in the 'select one or more files' dialog does not show a preview:

** (pitivi:5337): CRITICAL **: gst_video_info_from_caps: assertion 'gst_caps_is_fixed (caps)' failed
0:00:14.549058821  5337 0x7f124c045f20 ERROR                  libav gstavviddec.c:1059:gst_ffmpegviddec_negotiate:<avdec_h264-0> negotiation failed

after this, I can still import the files into the project, drag them onto the timeline, etc, but I get some additional errors, either during the putting on the timeline, or during scrubbing the timeline:

ERROR 14:54:44 pipeline             _asyncDoneNotReceivedCb: we didn't get async done, this is a bug (pitivi/utils/pipeline.py:404)
ERROR 14:54:44 pipeline             _recover: Pipeline error detected during playback, resetting -- num tries: 0 (pitivi/utils/pipeline.py:529)
ERROR 14:54:48 pipeline             _asyncDoneNotReceivedCb: we didn't get async done, this is a bug (pitivi/utils/pipeline.py:404)
ERROR 14:54:48 pipeline             _recover: Pipeline error detected during playback, resetting -- num tries: 0 (pitivi/utils/pipeline.py:529)
0:07:56.362160961  5337 0x7f1284008630 ERROR             aggregator gstaggregator.c:1376:gst_aggregator_event_forward_func:<smart-mixer-mixer:sink_5> Event seek event: 0x7f1230017630, time 99:99:99.999999999, seq-num 8295, GstEventSeek, rate=(double)1, format=(GstFormat)GST_FORMAT_TIME, flags=(GstSeekFlags)GST_SEEK_FLAG_FLUSH+GST_SEEK_FLAG_ACCURATE, cur-type=(GstSeekType)GST_SEEK_TYPE_SET, cur=(gint64)2098864194, stop-type=(GstSeekType)GST_SEEK_TYPE_SET, stop=(gint64)4167000000; failed
0:07:56.362387014  5337 0x7f1284008630 ERROR             aggregator gstaggregator.c:1393:gst_aggregator_event_forward_func:<smart-mixer-mixer:sink_5> Query seeking FAILED

and eventually/rapidly, pitivi will 'lock up' in that I become unable to scrub the timeline anymore