(3) If a person makes a vow to ה' or takes an oath imposing an obligation*an obligation Or “a prohibition.” on himself, he shall not break his pledge; he must carry out all that has crossed his lips.*crossed his lips Lit. “come out of his mouth.”
From Wikipedia: Jephthah (Hebrew: יפתח (Yiftāḥ), appears in the Book of Judges as a judge who presided over Israel for a period of six years (Judges 12:7). According to Judges, he lived in Gilead. His father's name is also given as Gilead, and his mother is described as a prostitute. Jephthah led the Israelites in battle against Ammon and, in exchange for defeating the Ammonites, made a vow to sacrifice whatever would come out of the door of his house first. When his daughter was the first to come out of the house, he immediately regretted the vow, which would require him to sacrifice his daughter to God. Jephthah then carried out his vow (This story is in the Haftorah for Parashah Hukat)
Dean: Do not make such broad vows because Jephthah made his vow too broad and he ended up having to sacrifice his daughter because he did not consider what might happen.
From My Jewish Learning, Comparing Vows and Oaths in Judaism: The Talmud states that the punishment for breaking a vow is the death of one’s children. The Shulchan Aruch explicitly warns people not to regularly make vows, and states that someone who does — even if they fulfill the vow — is called wicked and a sinner. Many observant Jews have the practice of saying b’li neder (“without a vow”) whenever they promise to do something to make explicit that they are not making a vow.
Ben: If you make a vow its like gambling. You can lose or gain somebody's trust by completing or not completing a vow you make. You are not sure when you make the vow if you can fulfill it or not when you make it.
From NY Jewish Week, "To Make a Vow is Human, to Cancel a Vow is Divine" by Rabbi Avi Weiss: What is fascinating is that in discussing the nullification of vows, the Talmud gives as an example a vow made by G-d. This occurs when G-d decides to destroy the Jewish people after they build the golden calf. Moses intercedes. The verb used for that intercession is va’yechal, the very term used in our text. As Rava in the Talmud said: “Moses stood before G-d until he cancelled His [that is, God’s] vows.” (Berachot 32a)
Sam: We are supposed to think of G-d like our parent. But, I would never have spoken like to G-d like Moses did to my mother. She would punish me for mouthing off! Moses was sort of mouthing off to G-d.