For me, it's special variables. Those things reeeally save me from messy situations. Setting up contexts, so I don't have to change much code.
Though funnily enough, I've seen someone abuse the hell out of them. For instance, there'd be code like this:
Code: Select all
(defun a_STZ () (declare (special a stz verb i ...)) (setf a 10) (b_STZ)) (defun b_STZ () (declare (special a stz verb i ...)) (list a)) (a_stz)
Until that point, /I/ didn't really know what DECLARE SPECIAL really did, and I had more Lisp experience than that programmer.