WordPress has a problem with incoming pings. If you write an article that has more than one link to the same blog in it, one or more of the pings may be discarded by their software. WP_PingPreserver can fix the problem.

You may wonder why the problem exists and you may also wonder why it's important to fix it. This is the problem, and I'll quote directly from Scott's article at Dammit Jim!:

…the function wp_throttle_comment_flood from wp-include/comment.php is automatically added as a default filter (in /wp-includes/default-filters.php). This function checks if the last comment from the commentor’s IP address was within 15 seconds and if so rejects the comment. This is obviously a problem with pings when 2 or 3 comments will be coming from the same IP within seconds.

I tested it on my blog to make sure it was a problem (and it was) and then installed the plugin. I did that last night. Today, I tested it by linking to Blog-Op three times. Yes, Chris was my guinea pig. Do you want to know what happened? The first link pinged the right article on his blog, the second link ping went into lala land and the third link pinged the second article on his blog. The third article did not receive a ping. I would definitely call this a problem. Fortunately, you can fix it on your own blog by installing the plugin. Hopefully, the WordPress developers will pick up on it and fix the core files for WordPress so we won't need the plugin.

Why is it important to fix it? Well, it probably isn't if you don't care about the people that link to your blog. If you do, read on as I expound on it.

All of the comment header links, which include manual comments, pingbacks and trackbacks, have the "rel=nofollow" tag added to the URLs on a standard WordPress installation. Those links count as nothing, as far as search engines are concerned, for people that comment or link to your blog. Many blog owners, including myself, have installed one of the many "dofollow" plugins to remove that tag and give link credit to everyone who comments or links to their blogs. Blogs that "dofollow" give link credit to every comment, pingback and trackback — link love at its finest.

So what happened when I linked three times to Blog-Op? Since Chris has a "dofollow" plugin installed, I received link credits for two out of three links. It should have been three out of three. It doesn't bother me, but I'm sure it would bother anyone else who links out all the time to other WordPress blogs when they see missing pingbacks on the other blogs.