Stress caused by an aging turbocharger or by a motorist increasing boost pressure from the turbocharger can overwhelm the spring that opens the wastegate at a specified pressure. In these case, the wastegate will flutter because the spring cannot keep the gate fully shut. The boost will oscillate and the engine will stumble and surge as the boost varies rapidly (any given throttle input should create steady boost levels). The engine may also respond sluggishly to throttle input since the wastegate is likely bleeding air.
