The Space Transportation System. The end of the Space Shuttle program is 4 or 5 flights away. I was just graduating from the University of Michigan when the first Space Shuttle was launched in 1981. Much has been written about Nixon's decision to press ahead with the Shuttle and the economic arguments that were based on projections of about 50 launches per year. That's a launch rate about an order of magnitude higher than what the Shuttle actually managed over the life of the program. Which means, of course, an order of magnitude larger emissions into the stratospheric each year.
By the time we appreciated how chlorine compounds and particles control stratospheric ozone, it was already clear that the Shuttle would never fly once a week. So global models for 50 launches per year were never run. How would 50 Shuttle launches per year affect ozone and climate? We will never know.