The last times i have noticed this lensman said it was because the fhg's are on rotating servers and that one of the servers didn't have the content on it so it 404'd.
An easy solution to this is to have your 404 pick up the ref code that was passed to it, and pass it along to whatever 404 adult.com has in place , this way if these errors happen again in the future the hits are still counted, instead of the way it is now . when a problem occurs the affiliate losing everything and adult.com loses nothing.
And another thing now that im venting
Cant you get the links in nats to pass along the webmaster ref codes as well. You wouldn't add webmaster links if you didnt get signups from them so why not let the webmaster get his ref code displayed along with the fhg's and regular links.
Not all sponsors do this , but i think its a great feature ( if you look at the sample sites in my sig you will notice the ref code is passed along to the webmaster links on the bottom of the pages )