I'm glad you brought this up, andabien, because I didn't realize I had a problem, too. When I type in a false page, I get my error document fine...or so I thought. I just tried typing in http://www.sassydevil.com/blue.php, and I get the "No input file specified" error. But those lines of code didn't work for me, and I tried a few different things. My error document is 404.php, so I tried that, among other things.
But I should also point out, I'm using WordPress, so I have a large number of RewriteRule settings. After those settings, I have this:
ErrorDocument 404 /index.php?error=404
ErrorDocument 403 /index.php?error=403
Options All -Indexes
Anyone know what I need to do? Thanks.