Bava Batra 22 - Town Commerce Restrictions
A tradesman who resides outside of a town in which his type of business has already been established can be barred from plying his trade in that town.
Rav Dimi, a Sage, brought his figs to a town which wanted to give him exclusive trading rights. They sent Rav Adda bar Abba to test him. He asked, "If an elephant swallows a willow-reed basket and then ejects it through its rectum, is it susceptible to ritual impurity or is it like manure? Rav Dimi didn't know, so Rav Adda patronized him. Rav Dimi's figs spoiled, and Rav Adda died. Every Sage involved took the blame for Rav Adda's death on himself - because they wanted to discharge their karmic debt.
One who builds a wall opposite the windows of his neighbor's house must ensure that the wall is either higher or lower than the windows, so that he can't stand up on it nor hang from it to look inside.