Accidentally Discovering A New Gas:
Carl Wilhelm Scheele produced an inflammable gas by immersing iron filings in dilute acid. At the time Scheele believed in the phlogiston theory, so he thought iron was a compound of calx and phlogiston. After producing the inflammable gas, he thought he had captured the myterious phlogiston, and called it "phlogiston elasticum", but in fact it was actually hydrogen.[1]