Special rules for redeeming and exchanging Maaser Sheni separated from demai: Bava Metzia 55b-56a
Tithing from some of a baker's loaves on other loaves of his, or some of a householder's loaves on other loaves of his, where there is a concern that some of his grain may have come from someone who had already tithed it: Bava Metzia 56a
Credibility of an am ha'aretz to testify regarding mixtures containing tithes separated from demai: Ketuvot 55b-56a
Using Demai
Selling demai, or giving it as a gift, to someone who might pass it along to someone who won't realize it requires tithing, causing him to err: Bava Metzia 56a