So long as there are no shifts, then spontaneous magic should generally do what you wanted it to do. The exact effects and manner in which it works may not be quite what you expected. Instead of a bolt of fire that incinerates your target instantly, he may just burst into flames, or his head may melt, etc.
When you start casting stuff you don't have the power to cast, then things start to go wrong. The more shifts, the worse it could be. One shift may turn that bolt of fire intended to kill one person into something that give second degree burns to him and the three guys he's with. (Or the other way around, killing one person when you intended to incapacitate all four.) With more shifts, the effects get more and more unpredictable. Instead of being able to communicate with one person mind-to-mind for the next hour, maybe everyone in the town becomes a shared hive-mind for two minutes.
If you start to get too many shifts, then the magic can start to "discharge". Brand new spells can be spawned that you never intended to cast. The more extra shifts you have, the worse these discharges can get.
Yes, it can be deadly. But that's why you need to be careful with what you cast. The system does tend to be unforgiving.
Not having started a new character lately, I have no idea what the "new player experience" is like, so I'm afraid I can't really help you with that.
