If you have joomla on the root domain (domain.com), and the url rewriting for deleting the index.php is activated... do you need the ErrorDocument 404 directive? Some example of this? It´s "ErrorDocument 404 /" useless?

But if you use the htaccess provided by joomla that rewrite the urls, joomla allow urls like that, removing the index.php. In that case, would be unnecesary I guess

Mmm I think that we aren´t talking about the same thing

I have this:

domain.com/index.php/sample/sampleton

if I access something like domain.com/sample I´ll get a apache 404, not a joomla 404. To avoid this, I do something like ErrorDocument 404 / or /index.php

Now I activate the rewrite and domain.com/sample it´s valid. I´m thinking that domain.com/dontexist will trigger the joomla 404 because you are already on the domain root. I have made some tests about it that looks right but simply put, it seems to work but I´m not sure why and if I´ll needed again later in some case I can see now.