The Mishnah continues with a declaration of a point of departure for the 2000 cubit walking limit.
IF A MAN WHO WAS ON A JOURNEY [HOMEWARD] WAS OVERTAKEN BY DUSK, AND HE KNEW OF A TREE OR A WALL AND SAID, ‘LET MY SABBATH BASE BE UNDER IT’,HIS STATEMENT IS OF NO AVAIL.
Why no avail?
Rabbah stated: What is Rab's reason? Because the man did not specify the exact spot.
The statement was too general. Because he did not take into account the width of the tree! One has to measure south including the northern part of the tree (ie through the tree trunk) and vice versa.
‘It was taught in agreement with Samuel’: If a man made a mistake and prepared ‘erubs in two opposite directions, believing that it is permitted to provide ‘erubs in two opposite directions, or if he said to his servants, ‘Go and prepare an ‘erub for me’ and one prepared for him an ‘erub in a northerly direction and the other prepared one for him in a southerly direction, he may proceed northwards as far as the limit of his southern ‘erub and southwards as far as the limit of his northern ‘erub.