A couple of days ago, I had problems with this blog, and found, after much searching, that my mySQL database had exceeded its quota; apparently, my hosting company just locked it, rather than sending me some kind of warning.
On this blog, I use a number of plug-ins to add features not available in WordPress, and wondered if a plug-in was the cause of the problems. The initial culprit seemed to be the Redirection plug-in, a tool used to manage 301 redirects for old pages that existed before I moved to WordPress. This plug-in also logs 404 errors, to help you find pages to redirect. The Redirection log table in my database was very large, and I deleted this to free up space.
But it turned out that Redirection was only doing its job, and the real culprit was the Sociable plug-in, used to share stories automatically on Twitter, Facebook and other social media platforms. It seems that there’s a file missing in the latest version of the Sociable plug-in (/wp-content/plugins/sociable/images/option1/32/more.png). The more.png file is missing in four folders, each for a different pixel size; I use 32px, so that’s the file generating 404 errors.
I solved the problem by pasting a 1px transparent .png file into the /32/ folder, and I no longer get 404s; there were some 208,000 404s for this file.
So, if you use the Sociable plug-in, you might want to check your 404 log. Certain other plug-ins that log errors, such as Redirection, may be logging these in your database, causing it to balloon. And no matter what, having lots of 404s in your logs makes it hard to find things when you need to sift through log files.