This metaphor simply means the person's going through a lot of different moods. "I feel the stench of failure coming on." Failure isn't fun but it doesn't smell. So, when this metaphor is used, it means one of life's disappointments is on the way. "This is the icing on the cake." While cake is always welcome, cake with icing is even better.
A cumulative exam means say that in December you're going to have to restudy whatever you learned back in September. Because that involves spacing, you're going to learn a lot from doing it. Spaced learning is especially important for long-term learning.
