Creatures with power or toughness calculated (such as Nightmare or Keldon Warlord) have their power/toughness overridden to be 0/2 so their normal ability to have their power/toughness recalculated won't do anything useful while the Sorceress effect is upon that creature. Same goes for Angry Mob. [bethmo 1996/01/30]
Note - This card was creature type "Sorceress" until Oracle 2007/10/01, when it became "Human Wizard".