WordPress MU

I'm seeing some odd errors in my php_error logs:

WordPress database error Duplicate entry '_transient_rewrite_rules' for key 2 for query INSERT INTO wp_3_options

So what causes WPMU to try to insert this information when it already exists?

I got it again, this morning, on a blog that wasn't being accessed by any of its owners.

So what fires this insert? As far as I can see from my apache logs its a post action from wp-cron.php