Alexander Fleming is given credit for accidentally (chance) discovering penicillin in 1928, but the antibacterial properties of penicillin was actually observed long before that. However the people who observed it before Fleming didn't understand what they had. In other words they weren't prepared to understand what they discovered.
Even after Fleming's discovery in 1928 it took 12 more years of research and development, and the help of another scientists, for penicillin to become the so-called "wonder drug" it is today.