One thing that would happen a lot would be magical accidents. People, especially wizards, would have to research new spells, and because they have absolutely no idea what they're doing they could very easily end up blowing their laboratory to smithereens, creating owlbears, or flooding a city with frogs that reproduce every time they croak. Or all three at once.
Sorcerers probably wouldn't have as much problem, as they're more instinctive, but they might easily produce some bizarre effects on their own, 'fooling around'. Psions and the like could easily unleash alien mindsets into the world (mindplagues). Clerics and other divine casters seem to be at an advantage here, because they're just conduits for some form of supernatural power, but they still might have accidents at first. (I had a discussion once about healing spells making people pop. It's the positive, healing energy that does it. I came to the conclusion that they had to develop filters and complicated spell procedures just to protect the people they're trying to heal.)
By an amazing coincidence, I'm actually working on a spell research system in my spare time, but it's in early pre-development right now. It's pretty free-form too, but I hope to have some specific guidelines. The basic idea is that wizards have to research spells at a certain cost, and in the process they create things that might not be such a good idea if you think about it. Like owlbears or demonic spirits. Or splitfrog plagues.