Had a teacher of mine in high school go off the deep end. Started talking strange, principal sat in on his classes later that day, was arrested and put in psych ward next day when found running down a major street in his underwear in freezing temps. Can happen out of the blue
I worked in a medical clinic with a contract doctor, a younger female who suddenly started behaving erratically. A sad case, and of course they had to let her go before she seriously harmed a patient.