I think a C in a 400-level course would seem to raise some flags. Typically, 300 and 400 level ourses are more understanding the material compared to 100 and 200 level, where it seems like it is just memorization and regurgitation. As well, I thought (I may be wrong though) students who make it to 300 and 400 level usually don't fail courses. In fact, at my school, there are very few students in those classes, and the class average is usually around a B/B+. Personally, I don't see a reason for ever getting anything below a C+, but it happens, everyone is different. But at the 400 level, there is really no reason to be getting grades below B, even if the course is incredibly hard. By the time you reach such advanced courses, your study skills should be at a level where you can even master subjects that may be beyond your scope at the beginning. There are jerk teachers, but that shouldn't make that much a difference. As well, a 90% fail rate, that class would be curved or the teacher would be relieved of the course responsibilities. So, to get back to your question, anything below a B at the 400 level, I imagine, would raise an eyebrow; but it shouldn't be the end all be all.