Just had a compare of my .htaccess & .htaccess from another thread and found what I've changed, it's the following lines

RewriteRule ^([^/]+)/favorites/(.*) user_favorites.php?username=$1&page=$2
RewriteRule ^([^/]+)/playlist/(.*) playlist.php?username=$1&page=$2
RewriteRule ^([^/]+)/friends/(.*) user_friends.php?username=$1&page=$2
RewriteRule ^([^/]+)/public/(.*)$ user_videos.php?username=$1&type=public&page=$2
RewriteRule ^([^/]+)/private/(.*)$ user_videos.php?username=$1&type=private&page=$2 [L,QSA]
RewriteRule ^([^/]+)/groups/(.*)$ user_groups.php?username=$1&page=$2 [L,QSA]

If you look I've removed the \. in the regex section of the ReWriteRule inside the [] brackets :-)