This is because you refreshed the upload page or clicked backbutton during video processing.

Here is the the web server log

Code:
76.64.64.161 - - [04/Oct/2007:17:08:59 +0000] "POST /upload.php?rnd_id=EQHcMnM5 HTTP/1.1" 200 - "http://video.bizhat.com/cgi-bin/uu_upload.pl?tmp_sid=f98ae029aad758dcc1dc2b8480e0a10a" "Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 2.0.50727)"
76.64.64.161 - - [04/Oct/2007:17:12:20 +0000] "POST /upload.php?rnd_id=EQHcMnM5 HTTP/1.1" 200 739 "http://video.bizhat.com/cgi-bin/uu_upload.pl?tmp_sid=f98ae029aad758dcc1dc2b8480e0a10a" "Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 2.0.50727)"
76.64.64.161 - - [04/Oct/2007:17:21:03 +0000] "POST /upload.php?rnd_id=EQHcMnM5 HTTP/1.1" 200 739 "http://video.bizhat.com/cgi-bin/uu_upload.pl?tmp_sid=f98ae029aad758dcc1dc2b8480e0a10a" "Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322; .NET CLR 2.0.50727)"
After upload finished, user will redirect to the page "/upload.php?rnd_id=EQHcMnM5" once.

From the Apache log, it seems you are visiting the page 3 times.

First time, the video get converted, if you try to go to that page again, since the video already processed, you will get that error.