Saeviomagy
Adventurer
Janx said:welcome to life.
they foiled your plan. You have 6 seconds game time to react. In the real world, your friends probably give you a little more time. But in the end, as a GM, I've got every right to give you a time limit and stick it to you. That's realistic. Wizards fumble and stutter in a fight, especially when they're on the front line. And especially when they're trying to do complicated stuff.
Janx
Well, probably the simplest one is "the monster moves and the spell becomes a very unwise choice" - AOE spells when the creature is in melee, targeted spells when the monster moves out of sight, that sort of thing.
The next situation that has happened is when a spell has been interrupted, and the next spell was to take advantage of it.
Oh, and note - I'm talking about quite quick decisions here, even when plans change. But it still remains that choosing a spell that's appropriate isn't nearly as fast as saying "I hit it with my sword".