Arrays of atoms coupled to waveguides can behave as mirrors. We consider an array of $\Lambda$-type three-level atoms wherein preparing the atoms in one ground state or another leads to reflection or transmission of the guided electromagnetic field; a superposition of the two ground states thus corresponds to a coherent superposition of mirror-like and transparent boundary conditions. We analyze the spontaneous emission of an excited two-level atom in the presence of such a quantum mirror, and inside a cavity formed by quantum mirrors, demonstrating that the resulting dynamics of the excited atom can exhibit exotic features, e.g., a superposition of Rabi cycle and exponential decay. Our results pave the way for exploring quantum electrodynamics (QED) phenomena in a paradigm wherein boundary conditions can exhibit quantum superpositions and correlations.