Overspeed. The common reasons for this are: 1. An altimeter is changed and so a function test and leak test are required but someone thinks that connecting the pitot isn't required because the airspeed wasn't changed. Applying a vacuum to the static port only, will operate the altimeter quite normally. But as the aircraft pitot is then kept at ground level, it starts to indicate airspeed as the relative pressure across its ports changes. You will hit the airspeed red line around 10 thousand feet of altitude yet most continue to the leak test altitude of 18000 feet plus. 2. Some aircraft have two pitot tubes but a common static system requiring that both systems be tested at the same time. Again consult the aircraft maintenance manual in all cases. 3. Another way this happens frequently is the various thin wall tubings often used in a PSTS kit. If the hose kinks due to the weight of the hose hanging to the ground or collapses the walls of the hose due to the differential pressure, you have severe problems you might not notice in time. Many of the latex tubings used to connect to the pitot mast collapse around 20,000 feet internal pressure with airport ambient altitude pressing and collapsing the thinwall hose. If you were supplied thin wall tubing throw it out and use aircraft hose. For the masts use thickwall and support the connection by taping to the body or wing.