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