Rashi: It is referring to the sin of not taking T'rumos and Ma'aseros from one's crops.

Targum Yonasan: It referring to eating Terumah and T'rumas Ma'aser, be'Tum'ah.

Yevamos, 89b: It is referring to the sin of separating from bad on to good - which must be effective; otherwise why would it be considered a sin?

Rashi: It is referring to taking T'rumos and Ma'asros from one's crops.

Rashbam: It is referring to desecrating Terumah and T'rumas Ma'aser by eating them be'Tum'ah.

Bechoros, 26b: It is referring to Kohanim and Levi'im who assist the owners in the granary and in the abatoir, and who accept T'rumos and Ma'asros in exchange. ¹

Yerushalmi T'rumos, 1:5: It is referring to people who eat from Ma'aser which the owner gave the Levi whilst still in its stalks - before the Levi has separated T'rumas Ma'aser. ²

Rashi: It implies that someone wo fails to separate the Matnos Kehunah will bear a sin', and 'someone who desecrates the Kodshim will die!' ¹