Text (5th+errata): 1/1. ; {Tap}: Target creature other than ~this~ becomes 0/2 until end of turn. [Oracle 2006/04/25]
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".