Dave McHale

– This plugin has been adopted! Please review the description below. –
A 404 page is what a user sees when they try to reach a non-existent page on your site (because they’ve clicked on a broken link, the page has been deleted or they’ve mistyped a URL).
While the standard 404 page can vary depending on your web host, it doesn’t usually provide the user with much useful information and users may just surf away from your site. Therefore, Google recommend creating a custom 404 page that provides the user with more information about your site and its content.
The Google 404 widget is a quick and easy way to embed a search box on your custom 404 page and provide users with useful information designed to help them find the information they need. Where it can, the Google widget will also suggest other ways for the user to find the information they need, thus increasing the likelihood that they’ll continue to explore your site.
For best results, you should set up a <a href="http://www.google.com/webmasters/" title="Google" rel="nofollow ugc">Google webmaster account</a> and submit an XML sitemap of your blog to ensure the Google 404 widget works effectively.
Find out more <a href="https://support.google.com/webmasters/answer/136085" rel="nofollow ugc">here…</a>
<h4>Features</h4>
Currently the plugin outputs the Google 404 widget in US English by default, or tries to use the current WordPress language. 
You can use the <code>google_404_language</code> filter to specify a different language code.
Also looking to add the possibility to <a href="https://support.google.com/webmasters/answer/100044" rel="nofollow ugc">change the appearance</a> of the widget.
<h4>Please note</h4>
The Google 404 widget plugin is NOT a WordPress widget. 
It is simply the term Google use to describe their enhanced 404 page script.
<h4>Adoption Story</h4>
This plugin was originally developed by <a href="https://profiles.wordpress.org/husobj/" rel="nofollow ugc">Ben Huson</a> and was adopted in September 2014 thanks to the <a href="https://wordpress.org/plugins/tags/adopt-me" rel="ugc">adopt-me</a> tag.
<h4>Coming Soon!</h4>
The ability to customize the CSS from an options page within the WordPress Admin area, attempt to dynamically inject the Google search box without needing to modify your template files, and more!

404 Widget for Google

Add Google search capabilities to your site's 404 "page not found" template.

Electric Code

If you create a custom taxonomy in WordPress, whether it’s part of a theme or a plugin, you may encounter a 404 error when trying to view the taxonomy on the front end of your website.
This normally only happens if you are referencing the taxonomy in your permalinks, i.e. if your permalinks look something like this:
/%category%/%postname%/
At first glance this seems to be fine, until you try to navigate to page 2 of the taxonomy archive. This will result in WordPress sending you to a URL something like this:
www.your_site.com/your_taxonomy/page/2
The ‘/page/2’ bit stuck on the end will clash with your permalink settings, causing in an a 404 error.
This plugin fixes that 🙂
See https://www.electriccode.co.uk/blog/fixing-404-errors-with-wordpress-custom-taxonomies/ for more information.
Usage
Step 1. Install plugin 
Step 2. Activate plugin 
Step 3. Enjoy your error free custom taxonomy pagination

Electric Pagination Fix

If you create a custom taxonomy in WordPress, whether it’s part of a theme or a plugin, you may encounter a 404 error when trying to view the taxonomy …

404 Widget for Google vs Electric Pagination Fix