Don’t tell it what not to do. Roughly, it doesn’t have the concept of ”not foobar”: mentioning such a negation in a prompt doesn’t do what a human would expect, and will instead cause ”foobar” activation and possibly also everything that is ”not” + ”foobar”, leading to inattention/off-task behavior as seen here.

You want your prompt to resonate with the desired output in a perfect harmony, and a ”don’t do X” is a bum note.