Disclaimer: I own nothing. That includes money, so don't bother suing. J.K. Rowling owns Harry Potter.
Plot Holes
Glasses
Why does Harry Potter have glasses? The Dursleys don't like him enough to even feed him on a regular basis. He doesn't have his own clothes; he wears hand-me-downs from Duddly. He does most of the chores, whereas Duddly does none. He does the lawn work. His birthday is not celebrated, nor does he get gifts at Christmas. In short, the Dursleys hate him, and try to pretend as much as possible that they have only one child to look after.
Since no one would really care if there was something wrong with him, he would probably not complain much. He got in trouble if he complained too much, anyway. If the Dursleys even knew that he was having trouble seeing, they would not have done anything about it. If he had told them, they would have thought he was only trying to get attention. Therefore, they would have had to find out from his school, after one of those annual vision tests. I find it unlikely that they would have taken it too seriously even then, but if they actually admitted to themselves that he wasn't faking it for attention, they would not have wanted to spend the time or the money to take him to get his vision tested, and get him glasses.
So why does he have them? In the book, he is supposed to be a small, inoffensive, and neglected/abused child. The taped up glasses add to that look.
A Possible Scenario:
"Mummy, Harry needs glasses!"
Petunia Dursley looked up, startled, from stirring the pot of vegetable stew she was making for dinner. Her sister's brat had recently been complaining that he needed glasses, but she had thought Vernon had put a stop to that when he had locked Harry up in his cupboard for three days without food. She wondered nervously if the little freak had somehow bewitched her sweet little Duddy-kins into asking for him.
"Why do you say that, Dudders?" she asked uneasily.
"Well, Mummy, I walked up to him and held my fist up, like this"-- the overweight six year old held up his small, pudgy fist in demonstration, "—and he asked where I had gotten a chicken leg, and if there was any left for him!" The boy's voice was shrill with indignation, and he seemed very upset.
Petunia sighed inwardly, but if it was something her Dudders wanted…
"Okay, Duddy-kins, I'll see about getting him an appointment after dinner," she agreed, resigned.
Duddly brightened instantly at the mention of dinner.
Thestrals
After Cedric's death, Harry was suddenly able to see thestrals. The problem is, Cedric was not the first person Harry had seen die. Normally, I would not have a problem with this, since he was only a year old when Voldie killed his parents, and could not remember it.
However, even though he can't remember that night, he can hear a woman—his mother— screaming when dementors are around. This is not consistent. If he can be affected by dementors and hear screaming because of that night, he should also be able to see the thestrals.
A Possible Scenario:
"Avada kedavra!" Lord Voldemort hissed. Lily Potter abruptly stopped screaming and crumpled to the floor, dead.
The toddler on the bed had had his eyes scrunched closed, wailing, so he had not seen this. When Lily stopped screaming, he opened his eyes and looked curiously at Lord Voldemort, who was wondering irately if his hearing would ever recover from the double onslaught.
Voldemort quickly raised his wand and said the spell a second time, before the baby could resume its screeching. The young Harry Potter blinked just as the green light flashed a second time, killing Voldemort instead of him.
Author's note: I don't have the books, so I'm not sure if the second 'scenario' is really accurate...
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