I go to admin/build/modules, without changing anything, and click 'Save Configuration'
In the database table menu_router it remove the access_callback value for items that had a value of either 1 or 'user_access'.
As a result, on pages that anonymous users should be able to get to, they get a access denied message (You are not authorized to access this page).
I was able to set the values in the database manually, but this really isn't a long term solution every time I go to admin/build/modules.
Today is the first day this started happening. I did change some menu items because I wanted to update their function names. After updating it, I went to admin/build/modules, then devel/menu/reset, then admin/settings/performance and cleared cache. I'm not sure what exactly could have been done to cause this.
In the file includes/database.mysql.inc, in the function db_escape_string, I had some custom code.
function db_escape_string($text) {
global $active_db;
if (!is_string($text)){ return '';} //custom
return mysql_real_escape_string($text, $active_db);
}
I commented out my custom code and it works fine now.