Be careful of what you vow to God - Judges 10-12.

Now Jephthah the Gileadite was a mighty man of valour, and he was the son of an harlot: and Gilead begat Jephthah. Then the Spirit of the Lord came upon Jephthah, and he passed over Gilead, and Manasseh, and passed over Mizpeh of Gilead, and from Mizpeh of Gilead he passed over unto the children of Ammon. And Jephthah vowed a vow unto the Lord, and said, If thou shalt without fail deliver the children of Ammon into mine hands, Then it shall be, that whatsoever cometh forth of the doors of my house to meet me, when I return in peace from the children of Ammon, shall surely be the Lord's, and I will offer it up for a burnt offering. So Jephthah passed over unto the children of Ammon to fight against them; and the Lord delivered them into his hands. And Jephthah came to Mizpeh unto his house, and, behold, his daughter came out to meet him with timbrels and with dances: and she was his only child; beside her he had neither son nor daughter. And it came to pass, when he saw her, that he rent his clothes, and said, Alas, my daughter! thou hast brought me very low, and thou art one of them that trouble me: for I have opened my mouth unto the Lord, and I cannot go back.
Judges 11:1,29-32,34-35 - hKJV