The device is not the internet.
Monotonous duties switch our brain to "rest mode", whether we like it or not, the researchers report in Proceedings of National Academy of Sciences.
They found mistakes can be predicted up to 30 seconds before we make them, by patterns in our brain activity.
The team hopes to design an early-warning brain monitor for pilots and others in "critical situations".
The scientists say the device would be particularly suitable for monotonous jobs where focus is hard to maintain - such as passport and immigration control.