Steps (starting from game save below, on a platform):
- walk backward to start rappel mode. If you don't release the key the game does not switch to climb mode, and you get a "rappel mode" messages loop going, even below the platform;
- release backward key when far enough from the platform that you immediately start falling
- press backward key while falling, this starts "rappel mode" messages loop again.
I've seen this repeatedly in some streams, and that's how I managed to reproduce the issue; But I'm not sure that's how it was triggered by the streamer. (I just asked and he said "I press S while in mid air", so we may never know)