The process of casting a spell consists of a series of steps that need to be followed in order. If at any point during this process a player is unable to perform one of these actions, the whole casting process is illegal and the game returns to the moment before that spell started to be cast.

For example, if a player begins to cast Sorin, Lord of Innistrad, but then realizes that he had no white mana to pay for its cost, the casting of Sorin isn’t legal, and the game goes back to the point before Sorin was going to be cast.

Similarly, if you try to use Go for the Throat to destroy a Porcelain Legionnaire, the spell would be targeting an illegal target, so the game rewinds as well. You made a choice, and that choice was illegal, so the game simply backs up to before you announced the spell. You aren’t forced to target your own creature (or any other creature) just because it would be a legal target. Also, the spell won’t be countered and “wasted” if you choose an illegal target. Choosing an illegal target during the casting process is different from having an illegal target at resolution; in the latter case, there’s no rewind, and the spell gets countered by the game rules.

Today’s Tournament Tip written by
Jorge Pinto, Level 1 Judge from Santiago, Chile