Today, I had a client tell me that after the upgrade to WordPress 3.8, some links were getting 404 errors. All of these links had the query string parameters of month and year. I noticed that when I removed the query string, the URL worked just fine. A little research told me that “year” was a reserved word that cannot be used in this manner. In fact, receiving a 404 error was to be expected in this situation. I changed my parameters to “evtmonth” and “evtyear” and everything worked as expected.
To learn more about WordPress reserved words, please see http://codex.wordpress.org/Function_Reference/register_taxonomy#Reserved_Terms