On what condition is the Navi Chayav Misah?
Rashi: If the prophecy, the sign or the wonder does not materialize. 1
In light of the current Pasuk, how will we explain prophesies of genuine Nevi'im that did not materialize, such as those of Yeshayah, who told Chizkiyah that he will die, and of Yonah who said that Ninveh will be overturned?
Rashbam: The current Pasuk is referring to positive prophesies, whereas the prophesies that did did not materialize were all negative prophesies, which can be overturned 1 with Teshuvah. 2
Meshivas Nefesh: The prophecy about Ninveh was that it will be overturned in forty days - if they do not repent until then. Alternatively, he said 'overturned', which can be for the bad (like S'dom was overturned) or the good (they will change from being Resha'im to Tzadikim) - in which case it did materialize.
Rashbam: Chizkiyah overturned his decree via Tefilah - See Melachim 2, 20:1-6, and Ninveh relented from their evil ways, and was saved.
What if the Navi issues, not just a prophecy, but a command that clashes with the Torah?
Rashi: We already learned (in Re'ei, in 13:4) that one is forbidden to listen to him and that the Navi is Chayav Misah - unless he is known to be a perfect Tzadik, 1 like Eliyahu on Har ha'Carmel, and he says to transgress only that once for a current need. 2
Ramban (in Pasuk 21): As long as he is an established Navi, he is believed to negate a Mitzvah once, if necessary.
Why does the Torah add "Lo Sagur mimenu"?
Rashi: It is a warning against failing to implicate him and not being afraid of getting punished for killing him. 1
Ramban: It is a warning not to be afraid of him on account of his wisdom or on account of the sign or the wonder that he will demonstrate'.
See Torah Temimah, note 76.
Who judges the Navi Sheker?
