Hmmm, That MD11 story.
Correct diagnosis is vitally important BEFORE any alternate data sources are selected. All airliners are equipped with a Standby Airspeed Indicator as well as the Capt and FO sources. As well, the Standby source is from basic raw data static and dynamic pressures. If a comparison was made with the Standby Airspeed it should be apparent which source is in error. However if alternate switching by one or other of the crew members occurs before this comparison is made then the crew is left with both Capt and FO airspeeds in agreement and depending on which source was selected both the CAPT and Fo Airspeed indications could be correct or both in error.