Yes, the one on my Geddy Lee has come off. It's the smaller one, that controls the tone. I put the bass into my gig back, took it out later on, and the knob had fallen off. I have no idea how. I also have no idea how to put it back on. The peg that the plastic knob goes onto is completely smooth, and thought they worked by being grooved, to grip the plastic top. I can click the knob back on, but it just spins freely. The actual metal rod bit still controls the tone, so I'm just at a loss as to how the damn thing gets back on.