Cached 404 not found pages sending 200 OK headers? Bad for SEO.

I’m having the same problem…

I found a solution that doesn’t require hacking the plugin. First, you must have a 404.php template in your theme folder, which should be located here:

/path_to/wp-content/themes/your_theme_folder/404.php

In that file, add the following to the top:

<?php
if (is_404()) {
    define('QUICK_CACHE_ALLOWED', false);
}
?>

This is processed before Quick Cache does its thing, and tells it not to cache 404 pages, which assures the 404 status code is always properly sent out. The ‘QUICK_CACHE_ALLOWED’ constant and instructions for using it this way was actually in the plugin’s read me file all along.

There’s a post about this on the official developer’s website that suggests putting the code above in a MU (must use plugin), but it didn’t work for me. Here’s a reference to the URL for those that want to read about it:
https://www.s2member.com/forums/topic/quick-cache-caching-404-not-found-pages/

I also came across another post of someone using another method to that optionally allows 404’s to be cached, while still sending out the 404 status code. I don’t much like that approach because it requires modifying the plugin code, which isn’t really future-roof. Here’s a reference to that for those that are interested:
https://techlogon.com/2013/01/06/how-to-prevent-soft-404-errors-in-quick-cache/

Just noticed that the 404 page sent a 200 ok header which is not cool at all :-/

@jumbo: Thanks for the solution, this did the trick!