Position the sight on a hard surface, use a business card to protect the finish.
Raise the rear of the slide so that the flat on top of the sight is parallel to the hard surface.
Use a flat faced punch and peen the sight with a downward tap. One, maybe two taps is all it should take.
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I am a big fan of green Loctite...add a small amount on top of the slide and it will wick underneath the sight.
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Try to fix the sight before shooting again. Once loose, the sight gets progressively worse after each shot.