Sure a wall-wart can have both positive and negative pins! XD I have one originally terminated on mini-DIN with +12, -12, +5, and gro--- uh, common. 
Granted, that's not what one typically imagines when thinking "wall wart", although I think discarding preconceptions when dealing with 'warts is probably not a bad thing. After all, is it center-positive? Center-negative? Regulated? 50% voltage rise under no load? AC??
Plugging in some random black box and expecting 5.0vDC on the center pin of a barrel connector is a good way to produce magic smoke.