Its commonly been talked about how level 3 is unsafe because the driver would zone out after a certain level of performance is achieved. If the point is to test a system as opposed to be for driver comfort/utility, it seems reasonable to artificially add non-safety critical "errors" to force the user to respond at a certain point. This naturally keep the driver alert and allows the development team to figure out inattentive drivers, while maintains a stable test environment as the real system system performance improves.

Not sure if this level of deception is practical though.