Gregor Mendel failed the teacher certification exam twice.
Mendel was working toward qualification as a certified high-school teacher while also pursuing scientific studies. In 1850, he failed the oral portion of the examination, which was the final part of a three-part process. His failure did not end his academic ambitions: in 1851, his abbot, Cyril František Napp, sent him to the University of Vienna for more formal study.
Mendel returned to the abbey in 1853 and worked mainly as a physics teacher. He tried the certification examination again in 1856, but once more failed the oral section. Despite never obtaining the formal teaching certificate, he became an influential teacher and researcher in Brno.
The setback is easy to misread as evidence that Mendel was a poor scholar. In fact, his studies with Christian Doppler and other Vienna professors helped prepare him for the pea-plant experiments that established the foundations of modern genetics.