This article describes a common issue when a vacuum line is accidentally broken on MINI Cooper models.
So how does the hose get broken? Good question!
Many times, when attempting to remove the air intake duct for the throttle housing, a technician may forget to disconnect the MAP sensor vacuum supply tube. Sometimes a tech may be making a thermostat repair, which requires removing the MAP sensor, and accidentally breaks the tube. Even age can play a factor in this scenario; although these tubes are made of some high strength material, they can become brittle over time. Just probing around and pushing on an old tube (item #20) could cause damage.