This part of the rules confuses me too much:
608.2b If the spell or ability specifies targets, it checks whether the targets are still legal. A target thats no longer in the zone it was in when it was targeted is illegal. Other changes to the game state may cause a target to no longer be legal; for example, its characteristics may have changed or an effect may have changed the text of the spell. If the source of an ability has left the zone it was in, its last known information is used during this process. The spell or ability is countered if all its targets, for every instance of the word "target," are now illegal. If the spell or ability is not countered, it will resolve normally. However, if any of its targets are illegal, the part of the spell or abilitys effect for which it is an illegal target cant perform any actions on that target, make another object or player perform any actions on that target, or make that target perform any actions. If the spell or ability creates a continuous effect that affects game rules (see rule 613.10), that effect doesnt apply to illegal targets. The effect may still determine information about illegal targets, though, and other parts of the effect for which those targets are not illegal may still affect them. #
Example: Sorins Thirst is a black instant that reads, "Sorins Thirst deals 2 damage to target creature and you gain 2 life." If the creature isnt a legal target during the resolution of Sorins Thirst (say, if the creature has gained protection from black or left the battlefield), then Sorins Thirst is countered. Its controller doesnt gain any life.
Example: Plague Spores reads, "Destroy target nonblack creature and target land. They cant be regenerated." Suppose the same animated land is chosen both as the nonblack creature and as the land, and the color of the creature land is changed to black before Plague Spores resolves. Plagues Spores isnt countered because the black creature land is still a legal target for the "target land" part of the spell. The "destroy target nonblack creature" part of the spell wont affect that permanent, but the "destroy target land" part of the spell will still destroy it. It cant be regenerated.