Discovery by accident is normal psychiatric science. Lithium came into use to treat gout, depression being seen a a "brain gout" [1]. The first MAO inhibitor was developed to treat tuberculosis, but doctors noticed that their patients were unusually cheerful [2] - and it was only synthesized in the first place because there was a lot of hydrazine left over from the V2 rocket programme!

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3712976/

[2] https://www.ncbi.nlm.nih.gov/pubmed/19442174