In a sealed bunker deep under the Pentagon, six soldiers remembered that their memories were all fake: they were an advance tactical team for an alien invasion.
They remembered nothing else — even their true aspect had been erased from their minds for security reasons. But they were, by human standards, geniuses, with none of humanity's psychological blind spots and cultural inhibitions. And they had very complete databases at hand.
It took the team two weeks to sketch a workable plan to trigger a self-reinforcing planetwide systemic disruption that would render effective defense impossible. At that point the people monitoring the experiment panicked. Through relatively less torture-like means than the ones they had used to instill on them the idea that their memories were fake, they programmed them with the fact that their knowledge of their memories being fake was the fake one.
The process went smoothly. While analysts pondered with horrified fascination the work of the pseudo-invaders, they were sent home to carefully hidden 24/7 psychological monitoring, which found no signs of regression or maladjustment.
There were no signs to find. Aliens were psychologically much harder to reprogram than humans, and immensely better liars. The team bid its time with inhuman patience.