Infiniti2000 said:The source is also a lack of a statement that allows you to change your actions once you've embarked upon them. For instance, you cast fireball only to have someone counterspell it with another fireball. Can you suddenly change it to lightning bolt and hope that your opponent doesn't have that, thereby negating his counterspell attempt (assuming no dispel magic either)? No, of course not, but there's no rule saying that you cannot change your action once you choose it and start it (that I recall anyway).
Brilliant Example