In Firefox and Opera, clicking on links within my blog will take me to the URL and load the page all right.
In Internet Explorer, however, when I click a link, the generic 404 page appears immediately (even though the URL exists, and I have a custom 404, anyway).
When I refresh the 404 page, the page loads in IE, too.
When I copy and paste the URL, it also loads fine in IE.
(Strange thing: when I load a page this way, the menu link that points to that page doesn't change color, as it should be with a visited link. As if the pasted URL and the clicked URL were different, even though they are perfectly identical.)
Tested on: Firefox 184.108.40.206 / Opera 9.2 / IE6
Permalink structure: /%post_id%/%postname%/
If someone needs more info on specifics, I'd be glad to give more details.