If your blog is running the SpeedPPC Dynamic Keyword Insertion plugin, it may be the cause of a mysterious and hard to trace 404 error - even on pages that aren't utilizing the dynamic keyword insertion feature.
My process for finding the cause of my random, mysterious 404 errors was:
1. Clearing all cache files
2. Re-saving permalinks
3. Turning off all plugins and then re-activating one-by-one (! important)
4. Clearing htaccess file of all code except that generated by a basic WordPress install
I skipped step 3 in previous problem solving attempts and was unable to find my error. Without step 3, everything would resolve during testing only to re-appear a few hours later.
According to the plugin developer, .htaccess rules common to popular caching plugins are "too complicated" and cause a random 404 error to appear on the site - even on pages that aren't utilizing the Dynamic Keyword Insertion feature.
Hopefully they will resolve this problem soon - and hopefully this post helps someone else.