Adding to a Mitzvah outside of the time-frame for performance of the Mitzvah: Eruvin 96a; Rosh HaShanah 28b
Adding for one Mitzvah alone, as opposed to fulfilling one item's mitzvah and adding to that of another within the same action: Rosh HaShanah 28b
Adding or subtracting by inaction: Rosh HaShanah 28b
Whether an act which violates this prohibition is legitimate in the event that it becomes necessary as part of a Mitzvah: Menachot 40b
What is the rule where the action is not carried out with intent to be fulfilling that Mitzvah to which he would be adding: Eruvin 95b-96a; Rosh HaShanah 28b; Menachot 41a
Where one is not responsible for a Mitzvah, and chooses to practice in anyway: Rashi Rosh HaShanah 33a "DePiturot"