Practical Prose of Harry Potter #33

Snape: “– mediocre, arrogant as his father, a determined rule-breaker, delighted to find himself famous, attention-seeking and impertinent –”

Dumbledore: “You see what you expect to see, Severus” (Harry Potter: The Complete Collection, 4050)

This is a perfect example of confirmation bias. We search for and favour information that confirms our prior beliefs. Before even meeting Harry, Snape assumed he would be just like his father. So, everything that he picks up on about Harry supports this assumption. This bias really warps the truth for Snape because us readers know that his characterization of Harry is inaccurate.