Page not found without record about page Is there a way to find the source of an error that I have on my production site during the last few days? In reports/dblog I see an error "Page not found" but there is no any information about what page, source, time of this error.

EDIT I looked in phpmyadmin and saw the source of the problem. The error appears when user reposts node in livejournal. I don't know is it a big problem for website or not (I mean problems with perfomance or search engines). Does anyone know the answer on this last question?
Is it something you're experiencing personally or are you just looking at the logs?

If you're just looking at the logs, it's usually a spammer/bot/crawler that's triggering the response. There are some security modules you can install that gives you a little more insight into those kinds of things (such as the IP address that triggered it), but I can only think of the D6 ones right now.