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In the Doctor Who episode "Blink", the Doctor tells Sally this about the Weeping Angels:
They are quantum-locked. They don't exist when they're being observed. The moment they are seen by any other living creature, they freeze into rock. No choice. It's a fact of their biology. In the sight of any living thing, they literally turn to stone.
Towards the end of the episode the Doctor then traps the four Angels by having them all facing each other after the TARDIS has dematerialised. If the Angels cease to exist while being observed, then how can they be observing each other while locked into stone? If the observation to trigger the quantum-lock counts as 'In the sight of any living thing', then they surely aren't actually trapped as when they're stone they don't exist so aren't observing one another, and so long as they move one at a time they could escape the formation the Doctor left them trapped in? Note that this question doesn't deal with how to generally defeat/damage an Angel in its quantum-locked state, but rather the very specific case and nuanced trap presented in this episode.