What terribly unwise vow did Jephthah judge of Israel swear to god?

ANSWER

He would sacrifice the first thing that came out his door to meet him

Judges 11 :

29 Then the Spirit of the LORD came on Jephthah. He crossed Gilead and Manasseh, passed through Mizpah of Gilead, and from there he advanced against the Ammonites. 30 And Jephthah made a vow to the LORD: “If you give the Ammonites into my hands, 31 whatever comes out of the door of my house to meet me when I return in triumph from the Ammonites will be the LORD’s, and I will sacrifice it as a burnt offering.”
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