Heh, the Overcasting system in Wheel of Time RPG was all kinds of neat. If you were out of weave slots, you could attempt to cast a 0, 1st or 2nd level weave, with increasing difficulty. You could also try to increase the level of a weave, up to 5 I think, but that was fairly difficult. Failure ranged from headache for the rest of the day to being burned out - losing the ability to cast weaves.
One of the reasons this had more of an impact than it seems, is because WoT has relatively few spells, but many spells have several levels of effectiveness, or you can improve the spell somehow by increasing the caster level. I.e. Gust of Wind (or something, haven't got the book with me), at the lowest level it can blow out a candle, at the highest level it can level buildings in a 10-mile radius. Also of note is the ultimate bad-ass spell Balefire, which not only utterly annihilate everything it hits, it actually annihilate the character or object back in time as well. Fire it off at the lowest level (8 or 9), and you burn 1 or 2 rounds back in time. If you crank it up to level 13 (note: you can only get up to weave level 9, so you'll have
to overchannel if you wanna use it at this level), you'll burn them out for months backwards in time.
I guess I should stop gushing about the WoT magic system, but it somehow hits all my buttons; I just love it