Chullin, Chapter 12, Mishnah 4. This chapter has been teaching the mitzvah of shiluach haken, sending away the mother bird before taking her young. Until now we have been asking when the mitzvah applies and how it is performed. Now the Mishnah turns to the person who did the opposite of what the Torah asked: he reached into the nest and took the mother herself. What happens to him?
"HaNotel em al habanim" - one who takes the mother bird while she is sitting upon her young in the nest. About such a person there is a disagreement.
The opinion of Rebbi Yehuda
Rebbi Yehuda's ruling is "lokei ve'eino meshaleiach": the man is liable to malkut, and the duty to release the mother no longer rests upon him at all.
His reasoning: the Torah said "Lo tikach ha'em al habanim", do not take the mother together with her young. The moment his hand closed around her, the prohibition was transgressed, and the transgression is finished. The Torah's instruction was to send her off first and only then take the young. Once he has taken the mother instead, releasing her afterward accomplishes nothing; there is no repair left to make. The obligation to send her away has passed out of existence. And so, according to Rebbi Yehuda, even if he does go ahead and release her, he still receives malkut for what he already did.
The opinion of the Chachamim
"VaChachamim omrim: meshaleiach ve'eino lokei" - the Chachamim say he must send her away, and once he does, he is not given malkut for having taken her.
According to the Chachamim, the command to release the mother was written precisely for a case such as this, where the bird is already in the taker's hands. The Torah's message runs this way: she is not yours to seize, and should you have seized her anyway, let her go, and with that release the sin itself is mended. The positive obligation is the remedy the Torah built into the prohibition, and whoever makes use of that remedy spares himself the lashes.
The general rule
The Mishnah closes with a principle: "Vezeh haklal". Any "mitzvat lo ta'aseh", any negative commandment, "sheyesh bah kum aseh", which comes attached to a directive to rise up and perform something, is a prohibition of which the Mishnah says "ein lokin aleha": no lashes are given for it. Should the offender carry out that attached deed, the earlier violation goes unpunished, since through his corrective act he has played his part in erasing the offense.
This category is known as a lav hanitak la'aseh: a prohibition that is "released" or untied by means of a positive commandment. The defining feature is that the positive command was given for the situation after the prohibition may already have been violated, and it functions as the way to fix the matter and remove the liability of malkut. Accordingly, when the person does the repair, no malkut is administered.