My graduate program was in Botany at a research botanical garden, so we pretty much only had required coursework in whatever the professors felt like teaching for any given semester, and all of the grad students that were pre-comps generally took everything offered for credit (usually two courses a semester). Usually it was something useful, like Phylogenetics or Plant Molecular Biology, but one semester, a professor who specialized in grass systematics decided he wanted to teach a semester-long course in Agrostology (the study of grasses) even though none of the grad students were particularly interested in grasses. So we started out as a not particularly motivated group in the first place, and were quite vocal about it (granted, this was probably not the most considerate thing to do, but it happened).
The professor didn’t really attempt to explain to us why we should spend a whole semester studying grass systematics: we were all interested in systematics in general, but it seemed unnecessary to spend the entire semester on one particular family none of us cared about. He then proceeded to go on a field trip for a month, leaving his grad student to teach the sections about the subfamilies he was interested in, and then came back to teach us the families she worked on.
Although all of this was quite annoying, it was not exceptionally demotivational. The real demotivation came when he gave us exams that seemed to be asking us questions about trivial details about grasses that were not systematically relevant nor clearly relevant to anyone’s grad research. The exams seemed to be primarily about whether or not we were listening in class, which really seemed to be condescending, on one level, and not intrinsically motivating, on another. I felt that, despite all of that, I actually had learned quite a bit about grass systematics in the class, and the fact that I got poor marks on (what seemed to me) irrelevant exam questions just made me angry and caused me to tune out for the rest of the semester.
I actually can still ID quite a few grasses, but I will always resent that class as being shockingly tone-deaf about motivating the students’ interest in the subject material and presenting relevant context for why the material was interesting or important at all.