A first contact mission to the Ghorusdans that ended in the deaths of forty-seven Starfleet personnel, including Captain Darson of the USS Adelphi, and for which Tam Elbrun was widely blamed.
The Ghorusda disaster is described in The Next Generation episode Tin Man. The Betazoid telepath Tam Elbrun served as first contact specialist on the mission and was held responsible when it went wrong. Two of the dead had been at the Academy with William Riker, which colours his attitude when Elbrun comes aboard the Enterprise-D.
Elbrun's own account is that he warned Darson about the Ghorusdans' cultural sensitivities and was ignored. An inquiry seems to have criticised him for not pressing the point harder. Data later observes that the captain's judgement was at least as much at fault.
The event is not dated on screen. Elbrun's reputation recovers little before he leaves Starfleet for good to join the living ship Gomtuu.