A few months ago, I imported a hosts file to block a few of the more annoying ad servers. It remaps their IPs to 127.0.0.1, as hinted here before. Netscape 7 doesn't like this, though, and will throw up an error message whenever it can't find one of those servers (sometimes 3 or 4 times per page!). Anyway, the easy way to stop that from happening is to turn on personal web sharing. Then Apache on your machine returns something that Netscape can deal with without an error message.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20021018064621623