Sanhedrin 5 - Permission to Judge
Although three judges are required for monetary cases, if the judge is a widely recognized expert who has successfully tried many cases without error, he may adjudicate these cases even alone. If, in addition, he has received authorization to judge from a government authority (The Babylonian Exilarch in the time of the Talmud) then the judge is free from liability if he rules erroneously.
It is thus understandable why one needs a permission to judge monetary cases. But why does he need it when judging the cases of ritual, provided that he is knowledgeable? Because of a story that happened when one student allowed to make ritually pure dough with egg yolk (מי ביצים, mei beitsim), and the people understood him as saying marsh water (מי בצעים, mei betsaim). After this it was instituted that one may not render decisions before he gets permission from his teacher, who will ascertain that he has clear diction and his rulings will be understood properly.
Art: Jean-Louis Laneuville - Portrait of the politician Bertrand Barère de Vieuzac