Why did Chavah add, "ve'Lo Sige'u Bo", seeing as Hashem did not say this?
Rashi: Indeed she added to the command, which resulted in her sinning. By adding, she ended-up subtracting. Hence the Pasuk writes in Mishlei 30:6, "Al Tosef al Devarav".
Sanhedrin 29a: We learn from here the principle, 'Kol ha'Mosif, Gore'a'. 1
See Torah Temimah, note 2, who elaborates. Citing Avos de'R. Nasan, he explains how, if Chavah added off her own bat the prohibition against touching the Eitz ha'Da'as, the Satan could have tricked her by pointing out that she did not die after touching it.
QUESTIONS ON RASHI
Rashi writes: "'And do not touch it' - She added to the command, therefore she ended up detracting from it." Why did she add to the prohibition? Also, when the snake pushed Chavah until she touched the tree (Rashi to 3:4), she could have simply conceded that they had never been commanded not to touch it?
Gur Aryeh: Chavah thought that Hashem commanded them regarding the Etz ha'Da'as, not as a Mitzvah, but simply for their benefit, due to the tree's harmful or even lethal nature. 1 She therefore thought that even touching the tree would cause death (and that Hashem had warned them only regarding eating its fruits because of their allure (3:6); whereas the trunk had no special attractiveness).
Ramban to 3:6 says this as well.
