What's the oldest "accidental (potion) ingredient as inciting incident" story?
Is it 1886's The Strange Case of Dr. Jekyll and Mr. Hyde, where the accidental ingredient's a "impurity of [unknown] salt"?
The TVTropes entry for this event seems to be a possible variant of what they call Accidental Discovery, Freak Lab Accident, or Miraculous Malfunction.
POSSIBLY FRANKENSTEIN (1818)
It can be argued that the hideousness of Frankenstein's creature was an accidental result of the creation process. The idea of the monster having been built with an abnormal brain is an invention of the movies, not the book, but there are elements in the book that could support the 'accident' interpretation.
Doctor Frankenstein builds his creature from raw materials supplied by "the dissecting room and the slaughter-house", but is forced to make it bigger than normal - around 8' tall - because it is too difficult to recreate the small details of a human body at their normal size. When the creature comes alive. he finds it horrifying to look at, with watery white eyes and yellow skin that barely conceals the muscles and blood vessels underneath.
It could be argued that the creature's horrific looks are an accidental result of using partly necrotic raw materials - the white eyes do suggest this. Or it could be an unforeseen, accidental side effect of the (unspecified) process that brings the creature to life. Whether this can be seen as an accidental ingredient is admittedly open to debate, but certainly, the result is very different from what Doctor Frankenstein intended, and thereon hangs the tale.
Behold! there is a medicine, potent, as a wise physician has assured me, and almost divine in its efficacy. It is composed of ingredients the most opposite to those by which thy awful father has brought this calamity upon thee and me. It is distilled of blessed herbs. Shall we not quaff it together, and thus be purified from evil?"
A case can be made for Nathaniel Hawthorne's 1844 story "Rappaccini's Daughter".
Without belaboring the whole story, young student Giovanni has fallen in love with Beatrice, the lovely daughter of Professor Rappaccini.
But because Rappaccini has exposed his daugher to the poisonous plants he has created, she herself has become poisonous. Nevertheless, Giovanni speaks with her regularly.
A rival professor, Baglioni, gives Giovanni an antidote that he says will cure Beatrice of her poisonousness.
But when she takes the antidote, it kills her.
Just at that moment Professor Pietro Baglioni looked forth from the window, and called loudly, in a tone of triumph mixed with horror, to the thunderstricken man of science, "Rappaccini! Rappaccini! and is THIS the upshot of your experiment!"
We can't entirely rule out the possibility that Baglioni knew the "antidote" would kill Beatrice, but it seems unlikely.
Nevertheless, as it turns out, Rappaccini had contrived to make Giovanni as poisonous as Beatrice, so that she coul have a mate and the doctor could continue his experiment, but Baglioni's "antidote" was the ingredient that spoiled all of that.
