So ATC cleared an aircraft for takeoff but on second thought cancelled the clearance and told him/her to hold short but that message was stepped on.
Now instead of telling the departing a/c to hold position on the runway and instructing the landing one to go around he just "hoped" the departure would become airborne in time? I think that's the point.
Anyway, probably a hundred of similar situations happen at the major airports a day, be it deliberate and in full awareness of everyone....