Like the following:

2013-11-07 10:57:25,165 ncsa.video ERROR : [527b7214e4b0f897ae5dd959] error processing [exit code=1]
[mpeg4 @ 0x2135740] Invalid pixel aspect ratio 2874/2873, limit is 255/255
Stream mapping:
Stream #0.0 -> #0.0
Stream #0.1 -> #0.1
Error while opening encoder for output stream #0.0 - maybe incorrect parameters such as bit_rate, rate, width or height
Traceback (most recent call last):
File "VideoPreviewExtractor.py", line 181, in on_message
create_video_preview(inputfile, 'mp4', '-1:720', host, fileid)
File "VideoPreviewExtractor.py", line 74, in create_video_preview
subprocess.check_output(['ffmpeg', '-loglevel', 'error', '-y', '-i', inputfile, '-vf', 'scale=' + size, '-ar', '44100', '-strict', '-2', previewfile1], stderr=subprocess.STDOUT)
File "/usr/lib/python2.7/subprocess.py", line 544, in check_output
raise CalledProcessError(retcode, cmd, output=output)
CalledProcessError: Command '['ffmpeg', '-loglevel', 'error', '-y', '-i', '/tmp/tmpR8kJjL', '-vf', 'scale=-1:720', '-ar', '44100', '-strict', '-2', '/tmp/tmpgToNSo.mp4']' returned non-zero exit status 1

Happens for some avi, mpg, mp4.

Also see
MMDB-1421