Despite making two attempts, he was unsuccessful in becoming a certified teacher

Sometimes humans don't get what humans want in life. This is demonstrated by Gregor Mendel, who made numerous unsuccessful attempts to obtain his teaching credential. Mendel worked as a substitute high school teacher in addition to receiving training to become a priest. He attempted the test to become a licensed high school teacher in 1850 but failed.

Mendel was instead sent to the University of Vienna for two years to take advantage of a new program of scientific teaching after failing an exam set by new regulations for teacher certification in 1850. Mendel spent his time in Vienna working with mathematical physicist Andreas von Ettinghausen and Austrian physicist Christian Doppler, just as he had done at Olmütz. Under the tutelage of botanist Franz Unger, a proponent of the cell theory and the developmental (pre-Darwinian) theory of life's evolution, he also studied the anatomy and physiology of plants as well as how to use a microscope. Mendel returned to the monastery in Brünn for the summer of 1853, and the following year he was again hired as a teacher, this time at the Brünn Realschule (secondary school), where he remained until he was chosen as abbot 14 years later. Despite the incident leading to a nervous breakdown and a second failure, he retook the teacher exam in 1856.