I’ll apologize up front for forgetting to write the mission and step down, but it’s when you are to find a way up on to the roof (a ladder) and then you use one of two mortars to rain destruction down on guards wandering around below. This is ‘before’ the rocket launcher part.
I could get on the ladder, and had no problem climbing up the ladder and getting off at the top. However, I changed my mind as to how I got ‘on’ the ladder (why? that’s my business! 
), and wanted to go back down and try to get on the ladder again (in my own double-secret probation mounting a ladder style).
Usually at the very bottom of the ladder, you can hit F again and you’ll get off. It would not let me off.