Eruvin 100B

דלא קא עביד איסורא, אבל הכא דקא עביד איסורא, הכי נמי דירד.

where one does not commit a transgression by refraining from action. However, here, where one commits a transgression every additional moment he remains in the tree, indeed, he should descend from it.

תני חדא: אחד אילן לח ואחד אילן יבש, ותניא אידך: במה דברים אמורים ⁠— בלח, אבל ביבש ⁠— מותר.

The Talmud cites an apparent contradiction: It was taught in one baraita that both a green tree and a dry tree are included in the prohibition against climbing a tree, whereas it was taught in another baraita: In what case are these matters, that one may not climb a tree, stated? With regard to a green tree. But in the case of a dry one, it is permitted to climb it.

אמר רב יהודה, לא קשיא: כאן ⁠— בשגזעו מחליף, כאן ⁠— בשאין גזעו מחליף.

Rav Yehuda said: It is not difficult. Here, the baraita that includes a dry tree in the prohibition is referring to a tree whose stump sends out new shoots when cut; whereas there, the baraita that excludes a dry tree from the prohibition is referring to one whose stump does not send out new shoots.

גזעו מחליף יבש קרית ליה! אלא לא קשיא: כאן ⁠— בימות החמה, כאן ⁠— בימות הגשמים.

The Talmud expresses surprise at this answer: You call a tree whose stump sends out new shoots dry? This tree is not dry at all. Rather, it is not difficult, as both baraitot deal with a dry tree whose stump will not send out any new shoots. However, here, the baraita that permits climbing a dry tree, is referring to the summer, when it is evident that the tree is dead; whereas there, the baraita that prohibits climbing the tree is referring to the winter, when many trees shed their leaves and it is not obvious which remain alive and which are dead.