I have a fondness for magic systems with an element of unpredictability; magic systems like that found in Cinematic Unisystem, Warhammer Fantasy Roleplay 2nd edition, both incarnations of Mage, etc. There have been a lot of musings in blogworld recently about systems to make magic more unpredictable/dangerous in good old D&D, many of which seem to have been inspired by the spell delay mechanic from Spellcraft & Swordplay, which, if I'm not mistaken, was inspired by the original Chainmail.
One of my old gripes with D&D's magic system is the relative safety of it. For the most part, spells have zero consequence for the mage, unless he tries to read a spell off a scroll that he is normally incapable of casting, or he uses haste on someone who is very old. After reading some of the excellent little subsystems out there, I had considered a system for adding variability to the magic system. (Both ways...spells that can occasionally go above and beyond the caster's expectations as well as spells that can go awry.)
Reordering my recent thoughts on Vancian magic, I'm not sure I need to do this for my current campaign. If Vancian magic relies on rote, it seems to me that it would be fairly safe. In Mage, there are no "spells"; every time the mage does something, he is literally warping the fabric of reality. Even an effect he has used before is a unique iteration of willpower and not a formula followed. In Warhammer 2nd edition (which has a very different magic system than the first edition), the wizard is taking hold of the Winds of Chaos and bending them into a spell. Granted, the spell is a rote itself, but the energy to fuel it is drawn from Chaos itself. D&D magic, by contrast, seems to be a bit more orderly. (Rigid is probably a better term)
Then again, just because you have a recipie doesn't mean the cassarole is going to turn out right... back to the drawing board, perhaps. I think I'll leave magic as written for my AD&D game, but future games I might have to tinker a little. The bottom line is this: I'd like for everyone to be just a little nervous when a spell is cast.