There have been a number of cases over the years of engines falling off 747s.
What surprises me about this is the high percentage of cases where this happens to a cargo 747F, when these are in the minority of the overall 747 fleet (and probably less intensively utilised as well). What is it about carrying cargo that seems to increase the chances of this ? It's not anything in the basic 747F design as a number where this has occurred were ex-passenger aircraft, and it has always been passenger versions that reached the highest number of hours.