 Nobs use a variety of standards to fit securely on potentiometers, encoders, rotary switches, what-have-you. In addition to shaft diameter, there's three basic shapes you're most likely to see. Smooth or round uses a simple cylindrical shaft. The knob is secured by a tiny set screw on its side, and yes, that screw is easy to lose and can come loose if not tightened fully. Neurled or split shaft are easy to spot with their rounded teeth. Be careful of subtypes which use different spacing between the teeth or splines. 18T is the most common standard, so go with that if you have to choose. D-shaft or flatted shaft is the most simple. Stabilized by the shaft's cutaway cylinder shape. As long as the knob's indicator line is placed correctly relative to the pot's orientation, you're good to go.