While most of her problems with Ferguson were pretty valid, there were a couple instances where she seemed like a bully. For example, in one episode, Ferguson seeks her for advice on how to properly date a girl he has a crush on. She deliberately gives him the most outlandish suggestions she can think of to sabotage his relationship with her. Granted, this backfires, but it's still a pretty mean and bullish thing to do.
There's also a dream sequence in one of the final episodes in which Clarissa kills Ferguson (really!), and he comes back as an angel and/or devil. They only hint at it ("the man upstairs" and "the great beyond"), but in the dream, he is dead, their parents fall apart (her father becomes a hippie who constantly mourns for his dead son and her mother takes up smoking) and Clarissa seems like she is barely concerned. This might be somewhat justified by it being All Just a Dream, but she acts nicer when she wakes up.