I picked this hardcover book up at an overstock store in the late '70s or early '80s and I haven't found the identification on Stack or other sources during sporadic searches in the past year. (The cover was an unremarkable beige.)
I remember two stories. In one, blurbed on the dust jacket, the genius/inventor is being shown some sort of government data repository where the information was secured by long strings of numbers. When he questions the security of the system he's told the odds of anyone coming up with a correct string are infinitely small, but somehow he guesses one and the whole system has to be redesigned.
In the other story I recall, he's trying to make some sort of cheap and plentiful food, but comes up with an addictive substance that makes people obese. Somehow he realizes the molecule turns unstable and explosive, but it's too late to stop its spread. I think the final line was something like, "And then someone sneezed."
My Google-fu has failed me.