Judges 11:30-40 explains this. Jephthah promised to kill and to give to God, whatever greeted him first. He expected an animal to greet him. However, his daughter greeted him.

We should be careful about gifts and promises. It is better not to carry out a promise, than to do something evil. But you should not make a promise that you cannot carry out. See also Matthew 5:33-37.

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