At the end of todays daf, 45a of Zevachim, the gemara brings a discussion of why we needed to learn the halacha of the korban, being that it is not relevant.
Tosafot quotes Reb Chaim who says that it is not relevant as a halachic decision because in the days of mashiach everyone will be tzadikim and will not transgress issurim. Will we lose our bechira in the days of mashiach and not have the ability to transgress?
Furthermore, is it not possible that while no kohain would commit a sin of pigul intentionally, is it not possible that a Kohain could err in the halacha and cause pigul by a mistaken thought?
Thank you
Rafi Goldmeier
Tosfos does not mean to say that nobody will transgress Isurim in the days of Moshi'ach.
Rather, in order to explain why there is no need to pass a Halachic ruling regarding Pigul today, Tosfos writes that such a ruling is unnecessary today because it us only useful under a combination of "uncommon" circumstances: First, it is pertinent only when Moshi'ach comes (and in the event that Moshi'ach himself will not rule on the matter first). Second, even after Moshi'ach comes it is only pertinent if a Kohen purposely or mistakenly makes a Korban Pigul, an event that will certainly be uncommon during the days of Moshi'ach - when the laws of Kodshim will be well-known to all practicing Kohanim.
This answers your questions; Tosfos does not mean to say that sin will be non-existent in the times of Moshi'ach.
Best wishes,
M. Kornfeld