That "in this case the oil pressure will be normal so it is likely just the gauge" case seems to be a standard thing that PPL students learn. I'm not convinced it's a smart thing to teach students. How likely is it and what is the chance of the pilot drawing the wrong conclusions because he remembers that the oil temperature gauge is very likely to fail?
Another case of rough running engine that most pilots are probably not aware of is mixture too rich. In hot and high conditions, you can often kill the engine by setting the mixture too rich. From my experience, students learn that in case of engine roughness, the mixture should be enriched.