What is the 'Hearback Error' phenomenon in air traffic communications, and what is the controller's critical duty during pilot readbacks?

1. A hearback error occurs when a pilot reads back an instruction incorrectly (or with an erroneous number), but the controller fails to catch the error because of expectation bias; controllers must actively listen to every readback to detect and correct discrepancies immediately.
2. When a pilot forgets to turn on the cockpit headset.
3. When an aircraft radio frequency suffers static interference from lightning.
4. When a controller speaks in an unauthorized foreign language.