"...that it can't be made worse by the addition of a headache" -- i,e, "it *can* always be made worse by the addition of a headache" -- to "IF you could add a headache to that"!
Thing is, that wouldn't hurt me none, 'coz I *already* have one, and in this case -- 1+1=1 !
(Yeah, yeah, sure; the headache was "just an example", right...? But... Whatever example you try, I can make *that* my original condition!)