In my 3e Freeport campaign, the players were shanghaied aboard a slaver ship bound for the Caliphates, which took an unexpected turn in bad weather and wrecked on the shores of an island near and dear to any old-schooler's heart - the Isle of Dread. The wizard was stranded without his spellbook, but soon discovered the book of another mage - ancient, weathered and heavily damaged, with some spells legible and others…not quite as complete.
"This time, for sure!" — B. MooseFor each of the incomplete spells, I prepared a short list of spells related in some way to the original spell - maybe a thematic or elemental relationship, with some spell effects lower than the level of the spell and some higher. Whenever the spell was cast, a die would be rolled to find out how the magic actually manifested. A successful roll on an appropriate skill or attribute (Spellcraft in 3e) will allow you to realize what form your magic is going to take in time to change targets (if necessary).
As an example, a partial fireball spell might look like this:
- lesser fire orb
- flaming sphere
- flame bolt
- wall of fire
- resilient sphere (they were out of fire, so you just get the ball)
While invisibility might be a bit different:
- disguise self
- improved invisibility
For added complexity, you could roll a Fudge die (if you don't have one handy, use 1-2 for a -, 3-4 for a blank, and 5-6 for a +) and give a further bonus or penalty based on that result. 3rd edition makes it easy - just choose a random metamagic feat to apply on a plus, and do the same but reverse it for a minus. For earlier editions, roll 1d4:
- A random aspect of the spell (range, duration, casting time, # of targets, etc) is doubled/halved.
- Caster is treated as being 1d4 levels higher/lower for purposes of the spell.
- Target gets a +2 bonus/penalty to save, if any.
- Other change (damage/element type, reversed spell, etc)
Since the results are random, tilt the tables slightly towards a net positive - especially if your player is being forced into using spells like this (like mine was). Even if they're using them by choice, they're giving up control and the ability to plan tactically for their effects. For the same reason, put some care into picking the effects - don't go for opposites (faerie fire for invisibility, for example) but for interesting twists that can be used in similar ways (glitterdust can blind an opponent as well as reveal an invisible creature)