-Any time they use the "soft focus" for a close-up of a woman (except perhaps when illustrating the effect of the Venus Drug).
If that's an embarrassing moment, you must be embarrassed by a whole bunch of TV shows from the 1950s and '60s. Using soft focus or diffusion filters for close-ups of actresses wasn't unique to Star Trek
Oh, I know. It's the longevity of the series that has made the effect embarrassing with age; it seems at odds with the modern sci-fi ethic, whatever that is. It doesn't bother me, but other people watching tend to laugh at the effect.