The Volkswagen Emissions Test Defeat Device needs no introduction:
Full details of how [the defeat device] worked are sketchy, although the EPA has said that the engines had computer software that could sense test scenarios by monitoring speed, engine operation, air pressure and even the position of the steering wheel.
When the cars were operating under controlled laboratory conditions – which typically involve putting them on a stationary test rig – the device appears to have put the vehicle into a sort of safety mode in which the engine ran below normal power and performance. Once on the road, the engines switched out of this test mode.
The result? The engines emitted nitrogen oxide pollutants up to 40 times above what is allowed in the US.
— BBC News 4/11/15
This perceptive post from cavmaths shows , I think, the danger of relying on widely used educational “best practice” short cuts. They can actually be deleterious to student understanding. In short, many of them are simply “educational defeat devices”, clever tricks designed to give a false impression of student performance under artificial test conditions, cheats that fall apart when tested in the real world.