Did you know that 404 pages are one of the most visited pages on websites? But what does your 404 page have? A simple 'Page not Found' on it? WHY??? You are wasting a good opportunity to get your product out there. If your visitors visit a 404 page, try to sell a product, give them an affiliate link to click on, get them to sign up to your newsletter. Believe me, it will make a big difference! Here is a blog post I wrote on Optimizing/Monetizing your 404 Error pages: Monetize and Optimize your 404 Error Page
I have been doing this for a while, I use it to get extra subscribers to my newsletter. It works, once in a while I get a few new subscribers. I don't get many, but I think it's worth doing. By the way, great blog post. - jens -
Or you could figure out why the pages are erroring out and fix them and redirect to the main content of your site. But I see no problem with having an affiliate link on a 404 page. While not exactly as targeted as you might like, it's a good idea that has been around a very long time.
A timely reminder to do this for all my sites. Even if you get one subscriber/ customer from doing this, then it's worth your while.