Improvised spells have been too safe to use - until now.
01-20 The strain causes caster to lose concentration. The spell is lost, as well as half (round up) the power points used to attempt the spell. 21-30 Mild mental lapse causes caster to lose spell and half the power points used. Caster is stunned for 1 round. 31-40 Moderate, but serious strain causes caster to lose both the spell and all the power points used in the attempt. Stunned for 2 rounds. 41-60 Serious mental lapse causes an ineffectual spell. One-fifth (round down) of the used power creates a random mutated spell into a random direction (80% chance) or upon the caster (20% chance). Caster is also stunned for 2 rounds. 61-75 Subconscious fear causes an ineffectual spell. One quarter (round down) of the used power creates a random mutated spell (as 41-60, above). Caster is also stunned for 3 rounds. 76-90 Severe strain causes caster to misfire. Stunned for 4 rounds. One quarter of the power creates a random mutated spell upon the caster, while another quarter of the power creates such a spell into random direction. 91-95 Extreme mental pressure causes caster to misfire (as 76-90, above) and collapse to the ground. Caster takes 10 hits, and is stunned for 6 rounds. 96-100 Caster internalizes the spell, taking from 1-10 random mutated spells at herself (power divided among these spells). Caster is knocked out for 12 hours and takes 20 hits. 101-125 Spell is mutated and launched into a random direction (roll d10; numbers 1 to 8 indicate compass directions, 9 is up, and 0 means the spell targets the caster). Caster is stunned for 1 round and takes 10 hits. 126-150 Spell is mutated, divided into two parts, both launched into random directions (as 101-125, above). Caster is stunned for 2 rounds and takes 15 hits. 151-175 Spell is mutated and is cast in direction opposite to the intended line (or, in case of benefical spells, affects the enemy instead of caster or a friend, etc). Caster suffers mental collapse, takes 25 hits, and is unable to function for 6 hours. 176-185 Caster internalizes spell, takes 30 hits, and suffers nerve damage in brain. Half of the power used rebounds from caster, launching from 1-10 random mutated spells into random directions. Caster loses all spell casting ability for 2 weeks, and requires 3 months bed rest (or nerve repairs, if available). 186-191 As 176-185, above, except that caster takes 40 hits, is in coma for 1 week, and loses all spell casting ability for 4 weeks. 192-195 Caster suffers a massive stroke, taking 60 hits; the spell is mutated into something harmful and targets the caster with its full strength, then the full power rebounds as 1-10 random mutated spells into random directions. Caster lapses into a month long coma. Caster will regain consciousness, but will die 3 hours later, unless major nerve repairs are done. In any case, caster loses spell casting ability for 6 months. 196-200 As 192-195, above, except caster takes 60 hits and is paralyzed from the waist down, not in coma. Spell casting ability is lost for 3 months. 201-250 As 192-195, above, except caster takes 50 hits, loses spell casting ability for 6 months and is in a coma for 4 weeks (won't die). 251-300 Severe nervous disorder causes the spell to be launched in harmful random mutated forms. In addition, every caster within 250' is ripped of power points (RR vs. the spell level to resist) which are also launched randomly. If the caster survives, he or she loses all spell casting ability for a year. 301+ As 251-300, above, except caster takes also 100 hits and dies quickly unless placed in stasis until given major brain repairs (or if brain is repaired immediately). In any case, caster loses all spell casting ability for a year.
Random mutated spell often resembles the intended effect, but may sometimes (20% chance) become almost anything. It's up to GM to say what happens. The worst fumbles can create permanent magic items or effects upon anything; sometimes these are even benefical.