Their procedure was to test each component before installation only, in order to prevent any potential damage occurring during post-installation testing. So they evidently couldn't know to scrap it and replace the damaged one. After the Mariner 8 failure, they changed this testing policy to test PCB's/components again after components are installed on the PCB.

"A diode intended to protect the system from transient voltages was thought to have been damaged during repairs/installation of the pitch amplifier's printed circuit board, something that would not have been detected through bench tests." - Wikipedia, but I'm having difficulty finding any original sources that validate or expand upon this.

P.S. I think you may have responded to the wrong parent comment

Edit: "N72-13963" was a report given to Congress in October 1971 on the failure of the Mariner 8 mission, I can't find that report but it may have more details if you're curious enough to track it down.

Edit: I found a transcript of the report given to Congress, evidently in June of 1971: https://books.google.com/books?id=7FQVAAAAIAAJ

Dr. Low: The diode was external to this integrated circuit, mounted next to it. I don't have one with me, but the diode is another component about the same size. The integrated circuit which failed is known to be sensitive to electrical transients if it is not properly protected. And electrical transients large enough to damage an unprotected integrated circuit — caused, for example, by engine start, stage separation, et. cetera — are well within the specifications of the Atlas/Centaur vehicle and are clearly possible.

Because of this, the integrated circuit was protected with Zener diodes which in effect were designed to act as "safety valves" to prevent potentially damaging electrical transients from reaching the integrated circuit itself. In accordance with standard practice in the Atlas/ Centaur vehicle — as well as in many others — these Zener diodes were checked individually prior to their installations, but were not tested thereafter. This practice of not testing protective devices after they are installed in a system is based upon the conclusion or design philosophy, which we are now reevaluating, that a test of a protective device after installation is not warranted where such a test could possibly damage the component that is being protected.

Based upon the detailed consideration of the flight failure records and some very extensive "detective work" of personnel from the Lewis Research Center, which is responsible for the Centaur stage, and General Dynamics, the prime contractor, we developed the following hypothesis which we believe pinpoints the cause of the Mariner 8 failure :

(1) One of the Zener diodes which was intended to protect the inte-grated circuit in the Centaur pitch rate gyro preamplifier was either installed improperly, was faulty itself, or failed at some time during assembly, vehicle checkout or powered flight.

(2) An electrical transient sufficient to cause damage to the inte-grated circuit occurred some time after T-minus-40 minutes—when the autopilot performed properly--but before Centaur main engine start.

(3) This damaged the integrated circuit affecting the gain of the autopilot pitch channel and causing the subsequent fixed engine position.

Tests show that all subsequent mission events would be fully explained by this hypothesis.

...

To prevent a recurrence of the problem which occured on Mariner 8, we required additional thermal and vibration tests on the Centaur state autopilot and new and special test procedures to ensure that the diodes protection the integrated circuits were properly installed and operative."