G.41.1- A spell may never target itself. [CompRules 1999/11/01]
G.41.Ruling.1- You may target a spell/ability which removes an ability at a permanent without that ability. It just does nothing. [Duelist Magazine #5, Page 23, 1995/05/01] This is unless the targeting restriction says it must have the ability, as with Phyrexian Splicer. [D'Angelo 2000/09/07]
G.41.Ruling.2- You may target a tapping spell or ability at a tapped card or an untapping spell or ability at an untapped card unless the card says it targets a tapped or untapped permanent. [Duelist Magazine #5, Page 22, 1995/05/01] See Rule G.38.3 and Rule G.43.3.
G.41.Ruling.3- Spells that target "attacking" or "blocking" creatures may only be used during the combat phase and only if there is an appropriate creature to target. [bethmo 1994/10/01]
G.41.Ruling.4- You cannot target a spell which will become a permanent with a spell or ability that targets a permanent until the permanent resolves. Prior to it resolving, it is just a spell. [bethmo 1994/10/01]
G.41.Ruling.5- Valid targets only include things in play unless the spell or ability specifically says otherwise. [D'Angelo 1999/02/22] For example, Unsummon can only work on a creature in play, while Raise Dead only works on a creature card in the graveyard.
G.41.Ruling.6- The pseudospell for an ability may never target itself. [DeLaney 1999/10/17]