We had a site that suddenly had an important menu item error out with a redirect loop. The odd part was we made no changes directly to the item. The main culprit for this is using the .htaccess but we hadn’t made changes to that file in months. There was also nothing in the Redirect Manager that would affect it. The issue ended up being that both the main menu item and another menu item that was set as menu alias had the same alias. It had been like this for probably a year, so our hypothesis is that something in a recent Joomla core upgrade caused it to no longer like that.
