Can the ECU Engine Control Unit on 1987 Honda Prelude have only one defect and car continue to operate?

No, a single defect in the 1987 Honda Prelude's ECU could prevent the car from operating altogether, or at least severely limit its functionality. The ECU controls so many critical engine functions (fuel injection, ignition timing, etc.) that even a minor fault could lead to a non-starting or poorly running engine.

While some minor glitches might manifest as drivability issues (rough idle, poor acceleration) rather than a complete failure to run, the ECU is the brain of the engine. A single, significant defect is unlikely to allow the car to continue operating normally. It would likely result in a noticeable problem, ranging from a minor inconvenience to a complete failure.