Maasros Chapter 5, Mishnah 11. This mishnah is a continuation of the previous one, in which the text of Viduy Maaser was explained - the declaration a person makes at the time of biur maasros, in which he testifies about himself that he separated the tithes properly.
"Asisi k'chol asher tzivisani":
The mishnah quotes the wording of the verse and examines it precisely: since a person testifies "Asisi k'chol asher tzivisani" - I have done according to all that You commanded me - this proves that the mitzvah was carried out in its proper order. Therefore, if a person separated maaser sheni before maaser rishon - not in the order in which they are meant to be separated - he cannot recite this declaration, since he cannot state that he did everything he was commanded when he did it in an incorrect manner.
"Lo avarti mimitzvosecha":
What does this statement add beyond the previous one? That the separation was done precisely according to its rules:
From a species for its own species: He did not separate Terumah and maaser from one species for another species. Someone who has grapes must take from the grapes, and he does not take from the grapes for the olives or the reverse. The Terumah and maaser must be taken from the very species one wishes to exempt.
From detached produce for detached produce: One does not take from produce that has been detached, which has already been picked and is therefore obligated in Terumah and maaser, for produce that is still attached to the ground. Produce attached to the ground is not yet subject to the obligation of Terumah and maasros, and so he would be taking from what is obligated for what is exempt, and that is not permitted. And likewise the reverse: one does not take from what is still attached, and therefore not yet obligated, for what has already been detached and become obligated.
From one year for its own year: One does not take from this year's crop for last year's crop, nor from last year's crop for this year's crop, for the verse in Devarim states that one must tithe the produce that comes out year by year, and not from one year for another. The separation must be from that same year itself.
"V'lo shachachti":
What is the meaning of "V'lo shachachti" - and I have not forgotten? That I did not forget to make a berachah and mention Your Name over terumas maaser. This refers to a particular berachah that is recited when taking terumas maaser, and the person testifies that he did not forget to recite it. Admittedly this berachah is only rabbinic, but it is attached here to the verse as an asmachta, since the verse contains a hint to it.
In summary: In this mishnah we learned how the Sages expound each of the phrases of Viduy Maaser: "Asisi k'chol asher tzivisani" - that the separation was done in its proper order, and not maaser sheni before maaser rishon; "Lo avarti mimitzvosecha" - that he did not separate from one species for a different species, nor from detached produce for attached produce, nor from attached produce for detached produce, nor from one year for another; "V'lo shachachti" - that he did not forget to make a berachah and mention Hashem's Name over terumas maaser.