Let it end in hellfire!
Henzell says...
  1. You are contaminated by 2(spell level^2)(badness)+500 and then damaged by 1d (spell level d (spell level + fail badness)/9) if you're contaminated by at least 800. For Air/Ice/Fire/Poison/Necro, the damage checks the corresponding resistance. Conj damag does not check AC, Earth checks AC 3 times. For Hex/Tloc/Tmut/Summ miscasts, see [2].
  2. These miscasts deal no damage. Hex miscast slows you for (damage) turns. Tloc applies -Move for (damage) turns. Tmut doubles current contam plus extra miscast contam. Summ creates a durable {nameless horror} with strength increasing with miscast severity.
  3. In 0.24-, failure to cast spells causes various generally nasty things to occur, becoming nastier with spell level and spell failure. Miscasts come in severity 0,1,2,3; 0 is always harmless. See {<school> miscast}.
  4. For actual miscast severity distribution as of the 0.25 miscast rework, see 'https://anydice.com/program/1b8a3. 'Follow the instructions in the comments there.

Information based on master branch and the great LearnDB.
© Dungeon Crawl Stone Soup, 2010.