מדרש על בבא מציעא 59:14

"and (you shall) ignore them": Sometimes you do ignore them and sometimes you do not ignore them. How so? If he were a Cohein and it were in the cemetery, (which a Cohein is forbidden to enter), or if he were an elder and it were beneath his dignity, or if his labor were greater than that of his neighbor, he is exempt, it being written "and you ignore": Sometimes you do ignore and sometimes you do not. (See #222).