The rules of the Soc. in question are laws which pretty much all Soc. abide by..and, you are asking a Q without any specific detail, as to why one was not allowed.. I would assume, a very good reason could be, for e.g.. a prof. competitor. for too many yrs..
I would also suspect this is a very rare occurence.. I know of only 2 such instances over the past 40 plus yrs ( and one of those was 6 yrs back and had not taken Prof exams the other was a part time " teacher " )