An amusing story about how Stefan Banach, a Polish mathematician and the founder of functional analysis got his PhD:
“He was being forced to write a Ph.D. paper and take the examinations, as he very quickly obtained many important results, but he kept saying that he was not ready and perhaps he would invent something more interesting. At last the university authorities became nervous. Somebody wrote down Banach’s remarks on some problems, and this was accepted as an excellent Ph.D. dissertation.
But an exam was also required. One day Banach was accosted in the corridor and asked to go to a Dean’s room, as “some people have come and they want to know some mathematical details, and you will certainly be able to answer their questions”. Banach willingly answered the questions, not realising that he was just being examined by a special commission that had come to Lvov for this purpose.”
via Math Overflow