Statically calling a non-static method from another class.
Abusing a language flaw that was only valid in PHP versions 5.0.0 to 5.1.2, see http://3...content-available-to-author-only...l.org/mAPH4.
Making the called method static won't work, see http://i...content-available-to-author-only...e.com/expLQq, the only viable solution seems to be using traits to clear this mess up: http://i...content-available-to-author-only...e.com/QABR3b
This would require PHP 5.4 or higher to be used, which you should be doing anyway...