Continuity mistake: During the car chase through the desert, the rear view mirrors on both cars disappear and reappear repeatedly.

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The Eraser - S2-E2
MacGyver: Dreams are important, otherwise sleep is just eight hours of nothing.
Trivia: As the series wore on, Dana Elcar began going blind as a result of glaucoma. This condition was eventually written into his character, as well.
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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.