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.