On the Content with Style blog today they take a look at some buggy functionality with parent:: they've found in the latest version of PHP, 5.3.3.
So, this app I hadn't been looking at in a few months did not work at all. I traced the bug down to a method that itself called a parent method. The parent only contains __call and __callStatic methods, and for some reason __callStatic was called, although the class it was called from was an object instance.
After using some sample code from the PHP manual, they spotted how things truly worked - the call is always sent to __callStatic when called from the extending class but not when called directly. Apparently they aren't the only ones that noticed the bug but noted that it was removed in PHP 5.3.4 to fix the issue.