Bechoros 24b - Davar Sh'eino Miskavein

There is a machlokes between the Teruma HaDeshen and the Magen Avrohom whether we can be matir one to do a פסיק רישא דלא ניחא ליה - an inevitable action where he isn't interested in the issur outcome, when the issur is only d'rabonon. The Terumas HaDeshen permits a פסיק רישא דלא ניחא ליה באיסור דרבנן and the Magen Avrohom forbids it (O.C. 314:5). R. Akiva Eiger proves that the Magen Avrohom is correct from a gemara in Succah.

Normally we pasken like R. Shimon that a דבר שאינו מתכוין is permitted, not like R. Yehuda who forbids it. However, when it is inevitable - פסיק רישא, even R. Shimon admits that it is forbidden as the gemara says 25a. When the gemara speaks about a p'sik reisha it is usually in a context where the person is interested in the issur outcome. But when the person isn't interested in the outcome - פסיק רישא דלא ניחא ליה, Tosafos in Kesubos 6a cites the Aruch who permits it even by an issur d'oraysa. Tosafos disagrees and holds that it is forbidden at least by an issur d'oraysa. The Terumas HaDeshen and Magen Avrohom argue if we can be matir by an issusr d'rabonon.

Tosafos explains in our sugya that even though Rav paskens like R. Yehuda who forbids a דבר שאינו מתכוין, that is when the issur is d'oraysa. But if the issur is d'rabonon (either because it is mekalkel, or because תולש לאו היינו גוזז), it would be permitted. It seems that a דבר שאינו מתכוין  according to R. Yehuda is worse than a  פסיק רישא דלא ניחא ליה according to R. Shimon. The Aruch holds that פסיק רישא דלא ניחא ליה is permitted, so even though we hold like Tosafos that it is forbidden, we see that there is grounds to be matir. It would be logical to assume that if Tosafos holds that even R. Yehuda would permit a דבר שאינו מתכוין when it is only d'rabonon, according to R. Shimon who is always matir דבר שאינו מתכוין, but is machmir for a פסיק רישא דלא ניחא ליה, he would at least be meikel when the entire issur is only d'rabonon. This would seem to support the opinion of the Terumas HaDeshen, against the Magen Avrohom. However, the M.B. writes that l'ma'aseh, most achronim agree with the Magen Avrohom to forbid a פסיק רישא דלא ניחא ליה even when the nature of the issur is only d'rabonon.