Yesterday I tried to access a page at Wolfram MathWorld and got this message instead of the material I was looking for:

This was puzzling since as far as I know, Firefox should only prefetch pages that the site specifically gives hints for. Apparently not.

What's more curious, however, is why Wolfram blocks the entire session rather than just using a rewrite rule that's triggered on the prefetch header to deny the prefetching itself. This would solve Wolfram's problem and not be so obnoxious for their users. Like so:

RewriteEngine On
SetEnvIf X-moz prefetch HAS_X-moz
RewriteCond %{ENV:HAS_X-moz} prefetch
RewriteRule .* /prefetch-attempt [L]

Today, it's not doing it. Maybe I just saw a transient problem.