You get a WordPress error that says, "HTTP Loopback Connections Disabled."
It took me awhile to figure this one out. I was getting it on a BuddyPress install (and other have reported seeing it with BackupBuddy, among others).
Most of the web sites had pointless or ineffective solutions. Some said to add define('ALTERNATE_WP_CRON', true); to wp-config.php, which was completely irrelevant, uneffective and annoying.
This is what worked for me.