The Crystalline Entity
The Crystalline Entity was number 89 of the holopod simulations available from Shari yn Yem during the drills she conducted aboard the USS Cerritos in 2381. It was based on another Starfleet crew's encounter with a Crystalline Entity. According to Yem, one was not supposed to approach the Entity during the simulation. Due to Yem lacking practical experience of starship emergencies, it apparently bore little resemblance to an actual Crystalline Entity encounter, and the scope was vastly underestimated; Yem remarked that the first Entity she encountered in person was larger than she had expected. (LD: "I, Excretus")