Worst case scenario
A worst case scenario was the worst possible scenario one could think of.
In 2372, Quark asked what was the worst thing that could happen if the warp drive on Quark's Treasure wouldn't shut down. Rom replied that "The ship could continue accelerating until it tears itself apart and scatters our remains halfway across the quadrant." (DS9: "Little Green Men")
An elderly Jake Sisko described his father's death as the worst thing that could happen to an 18 year old (him). (DS9: "The Visitor")
In 2374, Kira Nerys chided Odo for always bringing up the worst case scenario. The constable explained that somebody had to do it, and furthermore, everyone expected him to do so. (DS9: "The Reckoning")
In 2375, Dr. Julian Bashir admitted that the worst case scenario when he was linked to an unconscious Luther Sloan was that if Sloan died, so would he. (DS9: "Extreme Measures")