This entry was published at least two years ago (originally posted on November 26, 2020). Since that time the information may have become outdated or my beliefs may have changed (in general, assume a more open and liberal current viewpoint). A fuller disclaimer is available.
While spell development was superficially similar to most any other sort of creative process, the fine-tuning was killer. There were just so many variables to know what to adjust — ingredients, gestures, words, tone of voice — that those final touches could be quite dangerous.
Obligatory Wizard’s Bane:
https://www.fantasticfiction.com/c/rick-cook/wizards-bane.htm
https://www.baen.com/Chapters/201602FT1/201602FT1___3.htm
“Practice!” Moira snorted. “Perform a spell incorrectly and you may not get the opportunity to do it again. Look you, when those without the talent attempt a spell, one of three things will happen. The first, and far away the most likely outcome is that nothing at all will happen. What comes out is so far removed from the true spell that is it completely void. That is the most favorable result because it does no harm and it discourages the practitioner.
“The second thing that can happen is that the spell goes awry, usually disastrously so.” She smiled grimly. “Every village has its trove of stories of fools who sought to make magic and paid for their presumption. Some villages exist no longer because of such fools.
The third thing is that the spell is successful. That happens perhaps one out of every thousand attempts.” She frowned. “In some ways that is the worst. It encourages the fool to try again, often on a grander scale.”