One of ancient Israel’s leaders makes a rash vow that leads him to kill his own daughter.
There is the obvious lesson about making promises to God without fully thinking them out.
There is also the lesson that God’s blessings – even great and powerful blessings – do not always come with infallibility.
“The Spirit of the LORD came upon Jephthah” and God accomplished mighty deeds through him.
But then Jephthah makes a thoughtless vow and follows up by committing a horrible ontic evil.
Discernment, of course, is the answer: discernment in the things we do and the promises we make to God.