Factual error: MacGyver and Victoria go back to the location of the science event to think about the incident. MacGyver's friend, Bannister, suffers from a heart attack. MacGyver cuts a microphone cable and binds the cords to a metallic candlestick. MacGyver uses his creation as a some sort of defibrillator. Of course it works and MacGyver saves his friend. The maximum power in the wire is 11-48v DC with the maximum current of 10MA - not enough for giving huge electric shocks.
Continuity mistake: During the car chase through the desert, the rear view mirrors on both cars disappear and reappear repeatedly.
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Answer: Late one night, Geoffrey used a covert listening device to overhear David explaining his barricade device to Janet which is how he figured it out.
How do we know Geoffrey really did use a covert listening device? They didn't mention him using one.
There was a scene of him using it when David told Janet about his barricade.