Joseph,
Will any Jew ever sin, large or small, in the post-Moshiach and/or post-Third Beis HaMikdash period?
I hope not!
My understanding from what I’ve heard, is that after Moshiach comes and the Beis Hamikdash is rebuilt, the knowledge of G-d will fill the world as the waters fill the seas (Yeshayahu perek 11 and Chavakuk perek 2), so humanity and the Jewish people in particular will have a much greater awareness of Hashem, which will undoubtedly increase our fear of sin. Additionally, I’ve heard that our yetzer hara, which was altered to become a more powerful and confusing force in our psyches by Adam Harishon’s decision to eat from the etz pri hadaas, will be restored to the way it was in Gan Eden. This doesn’t mean that people cannot or will not sin, but it means that we will view sinful choices in a much more clearheaded light, seeing it as falsehood. So our fear of sin will increase dramatically, and the temptation and confusion of the yetzer hara that leads us to sin will be reduced dramatically.