A less common headset failure is really a frame failure. The head tube can stretch, allowing a headset cup, which is supposed to be a tight interference fit, to become loose in the tube. Lugless frames are most vulnerable; in a lugged frame the lug reinforces the top and bottom of the head tube and generally prevents stretching of this type. A loose cup can be fixed with a retaining compound such as Loctite 660, and some manufacturers produce slightly oversized cups to cope with this situation. It is usually the lower cup that is affected.