The Magid Mishna on Hilchos G'zela Va'aveda 1:9 quotes the rule that

מילתא דאמר רחמנא 'לא תעביד' - אי עביד לא מהני מידי

Something regarding which the Torah said "don't do [it]" - if one does it it has no effect.

(This topic is discussed in T'mura 4b.)

What does this rule mean and in what situations does it apply?

He explicitly contrasts this rule - which he applies in explaining the Ramba"m's position that there is no punishment for conspiracy to unlawfully obtain the possessions of another - with the case of forced sale. The latter is used elsewhere as an example of an illegal act that, once committed, has legal validity nonetheless. Furthermore, are there not other cases in which a person acts illegally but is still able to effect a legal effect, such as sales predicated on misinformation? There is obviously a distinction I am missing.

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    "3b" seems to be a misprint - it's actually Temurah 4b (bottom), the beginning of a long and celebrated discussion between Abaye and Rava on this very issue, whether אי עביד מהני (Abaye's position) or לא מהני (Rava's). The Gemara there goes on to cite a number of examples that go one way or another. – Alex Jul 17 '11 at 3:39
  • The topic of the question makes the question confusing. Your topic sounds like "Asur aval Patur" which sounds like Lehatchila vs B'dieved, but your question sounds like a completely different topic, and I'm not sure what you are asking. – avi Jul 17 '11 at 6:36
  • Thanks, @Alex for the correction, and @avi for the tip. I have edited the title to make it more clear to what I refer. – WAF Jul 17 '11 at 12:41

A prohibited marriage that is nonetheless halachically binding is an example that comes to mind.

This mishna is stating just that:

A prohibition of commandment: teachings from the words of the scribes. A prohibition of holiness:

  • A widow to the high priest;
  • A divorcee or subject of chalitzah to a regular priest;
  • A mamzer or nathin to a regular Israelite, or
  • A daughter of Israel to a nathin or mamzer

Mishna Yevamot 2,4

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  • Is this what you were looking for, or was it the other way around, or was it both? – David Perlman Jul 17 '11 at 11:37
  • Those are good examples of prohibited things being legally binding, which is the focus of my question - when is this the case and when is it not? Thanks to @Alex's correction of the citation, I now have a celebrated g'mara to learn through to shed more light on the topic. – WAF Jul 17 '11 at 12:45

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