Text (SH+errata): Rather than pay the mana cost for a spell, its controller may discard a card that shares a color with that spell. [Oracle 2009/07/08]
The effect applies to all players, not just the controller of this card. [D'Angelo 2006/05/23]
Only pays the mana cost (the mana in the upper right hand corner of the card). It will not pay additional costs from the card text (such as Buyback) or from other effects (such as Gloom or Chill). It does not prevent you from paying those additional costs if you want or need to. [Duelist Magazine #25, Page 30]
Discarding a Guerrilla Tactics to pay for a red spell through your opponent's Dream Halls will not trigger the Guerrilla Tactics second ability, because you are discarding as a cost, not an effect. [D'Angelo 1998/05/21]
You cannot target the discarded card with the spell you are casting. [D'Angelo 1998/05/28]
Vintage tournaments (see Rule 911) restricted this card from 1999/10/01 until 2008/09/20.
Legacy tournaments (see Rule 912) have banned this card since 1999/10/01.
Extended tournaments (See Rule 913) have banned this card since 1999/10/01.
Standard (Type 2) tournaments (see Rule 914) have banned this card since 1999/04/01.
