In the theatre, the reversal is a change in who is watching whom
The clearest example comes from a staging of Jean-Paul Sartre's Huis Clos, in which three characters are trapped together in hell. For the first half, the actors were confined inside a three-by-three-metre enclosure made of gauze, and the audience watched them the way a clinical psychologist might watch a patient through a one-way mirror [1][3]. The audience could see in; the performers were sealed off.
Then the gauze walls were lifted. The actors moved through the whole theatrical space, including areas close to the audience, and the spectators suddenly found themselves inside the action rather than outside it [1][3]. The reversal is not optical — no glass changes direction — but spatial and social: the people who were observing become the people being observed, or at least participants in the same room. The director describes this as the audience being forced to admit that "hell is the other" along with the characters [1][3].
In a device, reversal means switching the mirror state on and off
A different kind of reversal appears in a tristate electrochemical mirror device, which can switch between a transparent state, a greyish-blue state, and a mirror state [2]. Here the mirror effect is not one-way in the theatrical sense; it is a reversible optical state that the device can enter and leave. The abstract notes that non-dissolved nanoparticles help the electrodeposition step but impede the dissolution step, which affects how cleanly the device can switch back [2].
Switchable mirrors more generally are studied for their switching kinetics and reversibility, and the reversibility is strongly affected by the thickness of the layers involved [4]. So in this strand of research, "reversing" a mirror effect means making a coating or device that can go from mirror to non-mirror and back again, not turning a hidden observer into a visible one.
In acoustics, reversal means sending a signal back the way it came
Time-reversal mirrors in acoustics take a different approach again. A signal is recorded, reversed in time, and retransmitted, so that it refocuses back on its original source [5]. The paper discusses planar one-sided time-reversal mirrors and the physics behind them, including how the one-way wave equation is used in the modelling [5]. This is a reversal of propagation direction, not a reversal of who can see through glass.
Across these cases, the honest answer is that a one-way mirror effect can be reversed, but the word "reversed" carries a different meaning each time: a change in spatial relationships on stage [1][3], a switchable optical state in a device [2][4], or a retransmitted acoustic signal [5]. The theatre example is the closest to the everyday idea of turning the mirror around, and it works by changing the room rather than the glass.
Sources used in this answer
Spatial Dynamics in Jean-Paul Sartre’s Huis Clos: Otherness as Spatial Oppression
A staging of Sartre's Huis Clos used a gauze enclosure to create a one-way-mirror-like observation setup, then lifted the walls so the audience became part of the action.
A quasi‐solid‐state tristate reversible electrochemical mirror device with enhanced stability
A tristate electrochemical mirror device can switch between transparent, greyish-blue, and mirror states, with nanoparticles helping deposition but impeding dissolution.
Spatial Dynamics in Jean-Paul Sartre’s Huis Clos
The same Huis Clos production is described as reversing a one-way-mirror-like setup by moving from an enclosed, observed scenario to an open, interactive one.
High-throughput concept for tailoring switchable mirrors
Switchable mirrors are studied for switching kinetics and reversibility, which are strongly affected by layer thickness.
Time Reversal Mirrors and Cross Correlation Functions in Acoustic Wave Propagation
Time-reversal acoustics records, time-reverses, and retransmits signals so they refocus on the source, using planar one-sided time-reversal mirrors.