l'acharav u'ba lo l'acharav (Shabbos 92b)

The gemara explains that there are conflicting diyukim in the Mishna. As kollel iyun hadaf summarizes:

1. The Reisha exempts when he intended for in front and it went in back - but had he intended for in back and it went in back, he would be liable;
2. The Seifa is Mechayev when he intended for in back and it went in front - but had he intended for in back and it went in back, he would be exempt!

In that second case why are you chayav? If you are patur if you intended to carry it in the back and you did then you shouldn't be chayav just because it ended up in the front. At least not according to Rava. As kollel iyun hadaf explains the gemara on 72b:

ONE WHO INTENDED TO PERFORM A PERMITTED ACTION (a) If one intended to pick up something detached, and picked up something attached (Tosfos - he did not realize that it was attached; Rashi - he was trying to pick up something else), detaching it, he is exempt; (b) (Rava): If he intended to cut something detached, and he cut something attached, he is exempt; (c) (Abaye): He is Chayav.(d) Rava exempts, for he did not intend to cut something that may not be cut;(e) Abaye obligates, for he intended to cut in any case.

This is Rava talking in our gemara so doesn't it contradict his opinion from 72b? This morning someone showed me the Meromei Sade who answers this question. He says that when the gemara says that had he intended for in back and it went in back, he would be exempt it doesn't mean that he'd be patur had he done what he intended. It means that he'd be patur if he intended to carry it in the back on the side but it ended up in the back back because he intended for a good shemira but it ended up as a shemira pechusa. Had he intended for that shemira pechusa though he'd have been chayav. That would answer my question.