I had a strange error today. Lukily, a friend of mine warned me about it. I could hardly be aware of it without his help. (Thanks, Iosif!) So...

Why does Drupal 7 throws 404 error (Page not found)?

There could be plenty of reasons, but I guess the reason here is a bit stranger. All urls looked absolutely normal to me. I am using Pathauto with clean urls on. So why only SOME users get the page not found error? While looking at access logs, it seemed absolutely random to me. Of course, I had no 301 or 302 redirection component in case of errors.

I also didn't display a nice and friendly page for 404 errors (I thought I hadn't any). Pretty ignorant, huh? Visitors were discouraged and left away without even bothering to use the search box.