This is easy to reproduce.

joshua@emit:~/gst/gst-plugins/examples/indexing$ ./indexmpeg -q 1
/local/aleader/original/AlienSong.mpg
INFO ( 2830: 0) Initializing GStreamer Core Library version 0.5.0.1
INFO ( 2830: 0) CPU features: (0081813f) MMX
registry: loaded user_registry in 0.226414 seconds
          (/home/joshua/.gstreamer/registry.xml)
registry: loaded global_registry in 0.031504 seconds
          (/home/joshua/gst/gstreamer/registry.xml)
/pipeline/src.src: active = TRUE
/pipeline/demux.sink: active = TRUE
indexing /local/aleader/original/AlienSong.mpg...
/pipeline/demux: streaminfo = (GstCaps *) { application/x-gst-streaminfo;
mpegversion=(int) 1; bitrate=(int) 600000 }
/pipeline/src.src: caps = (GstCaps *) { video/mpeg; mpegversion=(int) 1;
systemstream=(boolean) TRUE; parsed=(boolean) TRUE }
/pipeline/demux.sink: caps = (GstCaps *) { video/mpeg; mpegversion=(int) 1;
systemstream=(boolean) TRUE; parsed=(boolean) TRUE }
/pipeline/src.src: active = FALSE
/pipeline/demux.sink: active = FALSE
/pipeline/demux.video_00: caps = (GstCaps *) { video/mpeg;
mpegversion=(int) 1; systemstream=(boolean) FALSE }
/pipeline/src.src: active = TRUE
/pipeline/demux.sink: active = TRUE
/pipeline/demux.audio_00: active = TRUE
/pipeline/demux.video_00: active = TRUE
/pipeline/demux.video_00: caps = (GstCaps *) { video/mpeg;
mpegversion=(int) 1; systemstream=(boolean) FALSE }
/pipeline/video_bin/video_decoder.sink: caps = (GstCaps *) { video/mpeg;
mpegversion=(int) 1; systemstream=(boolean) FALSE }
/pipeline/video_bin/video_decoder.sink: active = TRUE
/pipeline/video_bin/video_decoder.src: active = TRUE
/pipeline/video_bin/video_decoder.user_data: active = TRUE
/pipeline/video_bin/video_decoder.src: caps = (GstCaps *) { video/raw;
format=(fourcc) 'I420'; width=(int) 320; height=(int) 240;
pixel_width=(int) 1; pixel_height=(int) 1 }
/pipeline/video_bin/video_decoder: streaminfo = (GstCaps *) {
application/x-gst-streaminfo; bitrate=(int) 480000; framerate=(float)
24.000000 }
/pipeline/src.src: active = FALSE
/pipeline/video_bin/video_decoder.sink: active = FALSE
/pipeline/video_bin/video_decoder.src: active = FALSE
/pipeline/video_bin/video_decoder.user_data: active = FALSE
/pipeline/demux.sink: active = FALSE
/pipeline/demux.audio_00: active = FALSE
/pipeline/demux.video_00: active = FALSE

/pipeline/demux.video_00: caps = (GstCaps *) { video/mpeg;
mpegversion=(int) 1; systemstream=(boolean) FALSE }
/pipeline/video_bin/video_decoder.sink: caps = (GstCaps *) { video/mpeg;
mpegversion=(int) 1; systemstream=(boolean) FALSE }
seeking /local/aleader/original/AlienSong.mpg...
/pipeline/src.src: active = TRUE
/pipeline/demux.sink: active = TRUE
/pipeline/demux.audio_00: active = TRUE
/pipeline/demux.video_00: active = TRUE
/pipeline/video_bin/video_decoder.sink: active = TRUE
/pipeline/video_bin/video_decoder.src: active = TRUE
/pipeline/video_bin/video_decoder.user_data: active = TRUE
/pipeline/sink.sink: active = TRUE
/pipeline/demux: streaminfo = (GstCaps *) { application/x-gst-streaminfo;
mpegversion=(int) 1; bitrate=(int) 600000 }
/pipeline/src.src: caps = (GstCaps *) { video/mpeg; mpegversion=(int) 1;
systemstream=(boolean) TRUE; parsed=(boolean) TRUE }
/pipeline/demux.sink: caps = (GstCaps *) { video/mpeg; mpegversion=(int) 1;
systemstream=(boolean) TRUE; parsed=(boolean) TRUE }
/pipeline/sink.sink: caps = (GstCaps *) { video/raw; format=(fourcc)
'YV12'; width=(int) 320;
height=(int) 240; pixel_width=(int) 1; pixel_height=(int) 1 }
/pipeline/video_bin/video_decoder.src: caps = (GstCaps *) { video/raw;
format=(fourcc) 'YV12'; width=(int) 320; height=(int) 240;
pixel_width=(int) 1; pixel_height=(int) 1 }
/pipeline/video_bin/video_decoder: streaminfo = (GstCaps *) {
application/x-gst-streaminfo; bitrate=(int) 480000; framerate=(float)
24.000000 }

(process:2830): GStreamer-CRITICAL **: file gstdata.c: line 239
(gst_data_unref): assertion `GST_DATA_REFCOUNT_VALUE (data) > 0' failed
/pipeline/src.src: active = FALSE
/pipeline/sink.sink: active = FALSE
/pipeline/video_bin/video_decoder.sink: active = FALSE
/pipeline/video_bin/video_decoder.src: active = FALSE
/pipeline/video_bin/video_decoder.user_data: active = FALSE
/pipeline/demux.sink: active = FALSE
/pipeline/demux.audio_00: active = FALSE
/pipeline/demux.video_00: active = FALSE

i also did "b g_log" in gdb and got a stack trace:

Breakpoint 1, 0x404fdd8b in g_log () from /usr/lib/libglib-2.0.so.0
(gdb) where

+ Trace 32310

606               else {
607                 gst_pad_push (mpeg2dec->srcpad, outbuf);
608               }
609             }
610             if (info->discard_fbuf && info->discard_fbuf->id) {
611               gst_buffer_unref ((GstBuffer *)info->discard_fbuf->id);
612             }
613             break;
614           }
615           /* need more data */