In Goblet of Fire, Harry's close friends, and even best friend (Ronald Weasley), suspect him of sneakily getting past Dumbledore's age line and putting his name in the cup, and even when Harry repeatedly clarifies the truth, petty jealousy seems to trump their 4 year long best-friendship and everything Ron knows about Harry, such that he believes Harry is now lying about not having anything to do with the name being put in the cup.
I can understand why the majority of people think he tricked the cup, but his best friend, even after repeatedly clarifying?
Harry turned to Ron and Hermione; beyond them, he saw the long Gryffindor table all watching him, openmouthed.
“I didn’t put my name in,” Harry said blankly. “You know I didn’t.”
Both of them stared just as blankly back.
—Goblet of Fire, Chapter 17, The Four Champions“So,” he said, when Harry had finally removed the banner and thrown it into a corner. “Congratulations.”
“What d’you mean, congratulations?” said Harry, staring at Ron. There was definitely something wrong with the way Ron was smiling: It was more like a grimace.
“Well . . . no one else got across the Age Line,” said Ron. “Not even Fred and George. What did you use — the Invisibility Cloak?”
“The Invisibility Cloak wouldn’t have got me over that line,” said Harry slowly.
“Oh right,” said Ron. “I thought you might’ve told me if it was the cloak . . . because it would’ve covered both of us, wouldn’t it? But you found another way, did you?”
“Listen,” said Harry, “I didn’t put my name in that goblet. Someone else must’ve done it.”
Ron raised his eyebrows.
“What would they do that for?”
“I dunno,” said Harry. He felt it would sound very melodramatic to say, “To kill me.”
Ron’s eyebrows rose so high that they were in danger of disappearing into his hair.
“It’s okay, you know, you can tell me the truth,” he said. “If you don’t want everyone else to know, fine, but I don’t know why you’re bothering to lie, you didn’t get into trouble for it, did you? That friend of the Fat Lady’s, that Violet, she’s already told us all Dumbledore’s letting you enter. A thousand Galleons prize money, eh?
And you don’t have to do end-of-year tests either. . . .”
“I didn’t put my name in that goblet!” said Harry, starting to feel angry.
“Yeah, okay,” said Ron, in exactly the same sceptical tone as Cedric. “Only you said this morning you’d have done it last night, and no one would’ve seen you. . . . I’m not stupid, you know.”
“You’re doing a really good impression of it,” Harry snapped.
“Yeah?” said Ron, and there was no trace of a grin, forced or otherwise, on his face now.
—Goblet of Fire, Chapter 17, The Four Champions
In Order of the Phoenix, the wizarding community seems intent in rejecting that Voldemort has returned, so that includes accusing Harry (and Dumbledore) of lying.
I can understand why people who aren't close to Harry disbelieve him when he claims that Voldemort killed Cedric, especially when the Ministry wants to spread the false narrative that Cedric died in a tragic accident, but why do some of his friends, like Seamus, honestly believe that Harry would orchestrate such a lie about Cedric's death and Voldemort? It's not as if Harry had a reputation to warrant the strong suspicion (on the contrary).