Light Transport; Low Wing; 2 Turbojet Eng
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 13
Communication Breakdown
ATC Issue All Types; Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy
Human Factors
Aircraft X was a flight of 2 [light transport; low wing; 2 turbojet aircraft] and asked for a break up.  The other flight would be Aircraft Y.  I confirmed that the aircraft were vertically separated; [I] radar identified Aircraft Y and canceled MARSA between the two aircraft.  Aircraft X was assigned slow as [practical] airspeed and at FL250. Aircraft Y was assigned best forward airspeed and at FL240.  My intention was to take Aircraft Y down but misspoke and gave Aircraft X clearance to a lower altitude which descended him through Aircraft Y altitudes.  Aircraft X was turned to a heading of 320 until standard separation was established then given a fix back on course.I would not cancel MARSA so soon in the future.
I was the Supervisor on position at the time of the loss.I discovered the loss had happened after the fact from the Controller because I had just stepped outside the immediate operation to give her some feedback.  I asked if I could give her some feedback; and she told me she had just had a loss.  I had planned to tell her that she did a good job during her position relief briefing (an emphasis item.)What bothers me most about my perspective of this event is that I did not know it happened until after the event.  I do my best to remain engaged in the operation; listening to what is happening.  After finding out about the event; I replayed the time frame in my mind...what could have happened?About 10 minutes prior to her being relieved of her position; I noticed a Controller was standing next to her and assumed she was getting a break.  Then; I noticed the (assumed relieving) Controller had moved on to another position; again assuming she changed her mind.  She is a quiet controller.  I did not hear her when she asked another Controller to put in a flight plan because of a military break up (as I learned later). After talking to the Controller; the breakup was unexpected for her as well.  After finding out about the event; I found it difficult to believe I did not realize this break up had taken place.  If I had known; I most likely would have had the Controller that put in the flight plan stay until the break-up had finished.How could I have not heard this?  We were well-staffed on this morning; consequently; controllers were rotating through fairly frequently.  Looking back; I realized I was likely adjusting CruArt and a Controller asked about getting 1 hour of Time Off Award at the end of his shift.  So; I was adjusting CruArt and signing into Web Schedule to enter and approve leave for [another] Controller.  The area was well-staffed at the time; I believe we had 2 D-sides that were flexible to move to other positions (the traffic level was generally low for the area during this time).  Again; I remember seeing the Controller next to her but assumed he was offering a break; and then turned toward the computer to attend to the computer entries. (The computer faces opposite the control area.)In other words; I think it was job-related distractions that created my blind spot.I'm unsure if I could have done anything to prevent this loss; the Controller accidentally descended the wrong aircraft.  And upon review; the aircraft descended quickly.I know hindsight bias exists; however; if I had been aware there was a military break-up; I am certain I would have at least been listening and watching and most likely had someone help until the breakup was over.I guard against distractions of all types; however; this event has heightened my awareness that I am not immune to the distractions.  I will increase my vigilance in mitigating them as much as practical.  I have spoken to the Controller: she did well on the military break-up up until the inadvertent error.  I will also remain vigilant in providing an extra set of eyes and ears during military break-ups.
ZID Controller reports of flight break up and issuing descent clearance to wrong aircraft and then having a loss of separation.  FLM reports that he was distracted from the operation by doing other FLM duties and did not hear the Controller was having a flight break up.