page not found

Huh?

Hell, between cache clearings the Views menu link itself (under admin/structure) disappeared.

But actually the error was all me.

Wow.... it was the contextual filter i had added (thinking i was just adding it to the block and accidentally adding it to all displays).

If you give a page a default filter of use the content ID from the URL, and there is no content ID in the URL, rather than being the view page with no items you get page not found. Not the most helpful result from an easy-to-make (for me) error.

Seeing node and user pages with double slashes occasionally in my Drupal log as page not found.

Sometimes the referrer is listed as a page that redirects (i've got a crazy example.com/57 redirecting to node/67 thing going on in hook_init or somewhere unholy), sometimes a normal page, sometimes a page with even worse double slashes (which comes up page not found itself)

We had a site that occasionally generated page not found errors for ie6_fixes.css. Occasionally, we realized, because this file was only sought by browsers following the hide-from-everything-except Internet Explorer 6 code. The file was then not being found because the name of the whole theme had changed, and the existing template had hard-coded in not just the location but also the name of the theme.

Changed in page.tpl.php (and all variations such as page-front.tpl.php and page-node-4.tpl.php:
@import "/sites/default/themes/jonas_theme/ie6_fixes.css";