"Zoom creep" is a disease which affects some zoom lenses which change length whilst zooming. When the lens is held in a downward pointing direction, gravity is enough to cause the lens to extend thus changing the focal length. This can be a right royal pain in the backside when you have set your camera up for a particular location.
Better engineered zooms either a) don't extend (internal zooming lenses like the Sigma 100-300) b) have a clutch mechanism to prevent it from happening (Canon 100-400L and it's like) or c) are sufficiently well designed not to do it. (Canon 24-105mm etc.).