Mirror Universe Encounter
The Terran Empire insignia seen in the simulation
Mirror Universe Encounter was one of the holopod simulations presented by Shari yn Yem during the drills she conducted aboard the USS Cerritos in 2381. According to Yem, the simulation was based on the experiences of various other Starfleet crews.
In the simulation, the occupant was placed aboard a fictionalized mirror universe version of their starship, populated with mirror versions of their crew members, and tasked with escaping back to the prime universe.
Mariner arrives in the "mirror universe"
Ensign Beckett Mariner was among those that took the simulation. Despite bluffing her way past Lars Lundy, Migleemo, and Shaxs, Mariner was exposed by Brad Boimler when she gave the Terran salute with her left hand, as the Terran Beckett Mariner was right handed. The prime universe Mariner was immediately apprehended by Jet Manhaver and Dahae, then shocked by Boimler using his agonizer.
Unknown to the crew, Yem had tampered with the holo-program so that the entire crew would fail. (LD: "I, Excretus")