I've noticed some interesting things casting spells lately. When I cast a weakness spell, sometimes it worked on my opponent, but sometimes not. Instead, it might not work at all, other times it had the opposite effect: increasing the opponent's health over time. And once, it hit me instead!
Now, I think this is a great way for spells to backfire if cast on a powerful opponent. I've heard, however, that it is simply a bug. Please consider this a request for a (non-buggy!) version of these effects to be added to the game on spell failure. It will make magic much more interesting, assuming that one has a good idea to start with on what to expect with regards to failure or success.
That said, I still think that most spells should be a lot more powerful than they are if magic is going to be an effective part of the game.