When WordPress Posts Dis-appear?

[apondonet]

Occasionally my posts & comments dis-appear completely from my wordpress blog ( The first time this happened I thought someone had hacked my blog) - And I get a "Generic" "Sorry, no posts matched your criteria." or something of that sort.

After clearing my cache, cookies and re-starting the browser the posts "Miraculously" re-appear.

Question(s)

1. What causes this and how can it be remedied? If it happens to me occasionally, I am sure it happens to my users too - some who may not know what to do!
2. I have edited the 404 page(s) located in /wp-content/ themes/mytheme/ & /wp-content/ themes/default/ - This error page appears only when a URL is mis-typed and not when the posts have dis-appeared as I have outlined above - I get the generic error message instead, which is useless to my visitors.

What I am missing here? Is there some hidden 404 page somewhere?

[Kalina]

This only happens to me locally, it's never happened to me with a live site.