So instead of the usual “Whoops! We’re sorry but what you’re looking for has moved” page, consider customizing your error pages to get these people to do something.
For example, you could give them an option to opt into your list with an offer that is different than your landing page. This can be highly effective. For example…
“Congrats! You just found our secret page where we give away ___. Just tell us where to send it and it’s yours.”
Or you might make them an offer they can’t refuse, such as a great deal on one of your most popular products, or a combination offer for a super low price. Like this…
“Oops! You just landed on our error page, but we’re going to make it up to you. Here’s $xx.xx off of our super hot selling ___.”
For one reason or another, if you have your own website online long enough, you’re going to start getting a stream of people who land on a page of your website that does not exist, and leads them to a 404 error. Plan in advance for this hidden opportunity and you can turn some of the strayed visitors into subscribers and customers.