Continuity mistake: When MacGyver drives the forklift into the car one of the forks goes through the windshield. When the shot changes it is now out of the window. You can see the broken windshield where it had gone through.
The Prodigal - S1-E9
Continuity mistake: When the car breaks down and MacGyver backs up the tow truck he hits the Fed's car, but in the next scene after he gets out of the truck it is feet away, he never moved the truck.
A Prisoner of Conscience - S1-E21
Continuity mistake: When Pete steals the drugs, he puts them in his pocket and drops a bag right near the door, but when the doctor comes in and finds it it's much farther away from the door than where he dropped it.
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.