In the Anushasana Parva of the Mahabharata, just like in the Shanti Parva, Bhishma gives advice to Yudishthira and concerning how to be a good king and how to be good person, while he is lying on a bed of arrows after the end of the Kurukshetra war. In this chapter, Bhishma tells Yudhisthira about an ancient dialogue between Brahma and the sage Vasishta on the subject of what determines one's future, one's actions in the present birth or the destiny created by one's actions in past births. Brahma gives some examples to illustrate that it's a combination of both current actions and destiny:
In olden times, Yayati, falling from his high estate in heaven descended on the Earth but was again restored to the celestial regions by the good deeds of his virtuous grandsons. The royal sage Pururavas, celebrated as the descendant of Ila, attained to heaven through the intercession of the Brahmanas. Saudasa, the king of Kosala, though dignified by the performance of Aswamedha and other sacrifices, obtained the status of a man-eating Rakshasa, through the curse of a great Rishi. Aswatthaman and Rama, though both warriors and sons of Munis, failed to attain to heaven by reason of their own actions in this world. Vasu, though he performed a hundred sacrifices like a second Vasava, was sent to the nethermost regions, for making a single false statement. Vali, the son of Virochana, righteously bound by his promise, was consigned to the regions under the Earth, by the prowess of Vishnu. Was not Janamejaya, who followed the foot-prints of Sakra, checked and put down by the gods for killing a Brahmana woman? Was not the regenerate Rishi Vaisampayana too, who slew a Brahmana in ignorance, and was polluted by the slaughter of a child, put down by the gods? In olden times the royal sage Nriga became transmuted into a lizard. He had made gifts of kine unto the Brahmanas at his great sacrifice, but this availed him not. The royal sage Dhundhumara was overwhelmed with decrepitude even while engaged in performing his sacrifices, and foregoing all the merits thereof, he fell asleep at Girivraja. The Pandavas too regained their lost kingdom, of which they had been deprived by the powerful sons of Dhritarashtra, not through the intercession of the fates, but by recourse to their own valour. Do the Munis of rigid vows, and devoted to the practice of austere penances, denounce their curses with the aid of any supernatural power or by the exercise of their own puissance attained by individual acts?
Now many of these stories are recognizable: the story of Yayati falling from and going back to Devaloka described in the Adi Parva of the Mahabharata, the story of Pururavas and Urvashi, the story of Saudasa turning into the demon Kalmashapada, the story of Parashurama, the story of Vamana defeating Mahabali, the story of Vaishampayana incurring the sin of Brahmahatya discussed in my answer here, and the story of Nriga turning into a lizard discussed in my answer here. And there's the story of the Pandavas defeating the Kauravas, which is interesting because it suggests that the Mahabharata war happened before in an earlier age and Bhishma knew the outcome.
But my question is, what is Brahma referring to when he says "Was not Janamejaya, who followed the foot-prints of Sakra, checked and put down by the gods for killing a Brahmana woman?" Who is this king Janamejaya?
I suppose it could be Arjuna's great-grandson Janamejaya; if Brahma is describing a previous iteration of the Mahabharata then he could be referring to the Janamejaya of that iteration. Or it could be one of the other kings with the name Janamejaya who are mentioned in the Puranas and the like. Does anyone know if any scriptures mention a king named Janamejaya killing a Brahmana woman?