On the first day of second-year Herbology, a boy named Justin Finch-Fletchley introduces himself to Harry. He starts chatting about his family ["My name was down for Eton, you know […] Of course, Mother was slightly disappointed, but since I made her read Lockhart's books I think she's begun to see how useful it'll be to have a fully trained wizard in the family" (6.67)]. Of course, by saying that Justin's family was planning to have him attend Eton (a famous British boys' boarding school), Justin is also telling Harry that he's Muggle-born.
The fact that Justin is Muggle-born wouldn't matter at all normally. After the attack on Mrs. Norris, though, Justin becomes one of the many Hogwarts students convinced that Harry is the Heir of Slytherin. When Justin sees Harry coming down the hall, he "turned abruptly and sped off in the opposite direction" (9.66). Justin shares his fears with his friends in Hufflepuff. After Colin Creevey is Petrified, Harry overhears Ernie Macmillan (another second year) announcing: "So anyway […] I told Justin to hide up in our dormitory. I mean to say, if Potter's marked him down as his next victim, it's best if he keeps a low profile for a while. Of course, Justin's been waiting for something like this to happen ever since he let slip to Potter he was Muggle-born" (11.135).
Naturally, it only makes it look worse for Harry that the next person Petrified is Justin Finch-Fletchley, along with Nearly Headless Nick. The speed with which Justin goes from friendliness to fear emphasizes the power of fear to overwhelm people's judgment. Still, we have to give Justin credit: as soon as the Mandrake Restorative Draught works and he wakes up, he "[hurries] over from the Hufflepuff table to wring [Harry's] hand and apologize endlessly for suspecting him" (18.129).
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