Improper 404 error handling

I have a problem with 404 error pages in WordPress 2.6. When I try to load a post or wordpress page that does not exist, it correctly uses my theme’s 404 template. But when I try to load a file that does not exist it simply displays my front page.

So loading /robots.txt is OK, loading /robots2.txt should give a 404 error but doesn’t.

Now this problem only occurs when I do not use permalinks. When using the index.php?p=17-format, nothing’s wrong. When I change that to something pretty, every time Apache should throw a 404 it just loads my front page. Adjusting the settings of my static front page has no effect. My ErrorDocument directive in my .htaccess file is ignored. Note: every time WordPress should throw a 404, it does so. No problems there.

I have browsed the web for solutions and found some — but only for WordPress 2.5. I understand should have been solved in 2.6 (which I use) but I cannot for the life of me get it to work.

Do you guys have any ideas if there is a fix for this issue? Am I doing something wrong? Any help would be much appreciated.