I don't own the beginning of this chapter. It was written by the wonderful Meteoricshipyards in the story "Luna's Hubby", and is used here with permission.
Many books have been written about magic. How it works, why it works, why it doesn't work for Muggles, why there's an Occulus Reparo spell and a generalized Reparo spell, why complex magic never seems to work exactly as you expect, and so on. The relationship between magic and intent is, well not explained, but expounded upon (or more correctly - obfuscated) in some very large tomes. It would be impossible to go into it in any depth here, but the short and sweet answer is that the more complex and powerful the spell, the more likely it is to not do what you want, especially if what you want isn't what you said. In other words, if what you said wasn't what you meant, then the chances of getting what you want are vanishingly small. The best reference for this is Murphy's Magical Laws and is found in most well stocked magical libraries. Usually misfiled.
The application of this bears directly on the events related here, because the wards around Privet drive were set up with a certain intention, and were complex and powerful. Dumbledore set up the wards as an extension of the protection given Harry by his mother. They were supposed to keep him safe. Whether they would have protected him from Death Eaters is a moot question, since they never found him and didn't get a chance to test the wards. What is well known to the police and Harry's new family is that they didn't protect Harry at all from the pain inflicted by his relatives.
Dumbledore tried to tie the wards into the idea of home and family and would say that as long as Harry had a home with the Dursleys the wards would protect him. What he never said to anyone, considering it of no importance, was that it took two tries to set the wards up. That may be because it was an extremely difficult and complex spell. Or it could be that the idea of home and family, when talking about Harry and that family, wasn't a concrete enough idea to anchor a cantrip, let alone a powerful ward. In the end, what Dumbledore built the wards on was the legal definition of home, and the sense of protection. Not that he knew, since that wasn't his original intention.
One summer day, that definition was changed. And the wards fell.
The Dursley's had decided that it was a wonderful day to go to the zoo. As it happened though, they had no one to watch their young nephew. He couldn't go to the zoo with them of course, but they also couldn't just leave him at home. The boy was three years old, who knew what could happen?
Eventually, the family decided to take him with them, and leave him in the car. This was what they did, not knowing how that one decision would change the future.
Maria Duncan and her husband Stephen had decided that it would be a wonderful day to take their children to the zoo. They hadn't been in a while, and there was a new exhibit. Of course, Luke was only two, but he could still enjoy it with them somewhat, and six year old Elizabeth was buzzing with excitement.
Stephen Duncan parked the car, and he and his wife took their children out. Elizabeth smiled happily and began skipping around the parking lot. Soon, she started looking in the car windows, a habit common for her. Maria sighed, heading over to the girl, intending to chastise her. She was glad to see her daughter so curious, but this was inappropriate. However, she never got the chance as Elizabeth spoke first.
"Look, Mum! There's a little boy in this one." Maria's eyes widened, and she stormed over to the car. Stephen, noticing his wife's anger, quickly grabbed Luke out of his car seat and headed over to her.
"Stephen, look! There's a boy in this car. He can't be any older than Luke!" Stephen tried to calm his wife, knowing her tendency to act rashly.
"Maria, I'm sure his parents are right around here. They're probably just going to get year-long passes or something else that would take just a minute. He's asleep. I'm sure they thought it would be easier to leave him for a minute and come back."
"I don't care. That's neglect! I'm calling child services."
"Maria, wait. Calm down. One of us might have done the same thing, maybe not with the kids so young, but it's not unreasonable. Why don't you just wait a minute and see if they come back."
Maria agreed, albeit reluctantly, and stood by the car, fuming. They waited for fifteen minutes before Elizabeth and Luke got bored. Stephen took them over to a grassy area to play, while Maria continued to wait.
Finally, they realized that no one was coming to get the child. Maria, in a blaze of righteous anger, prepared to call the police. She did not know the number of Child Protective Services off-hand, and she figured that was who she should call instead. Stephen wrote down the license plate number of the car while Maria made the call.
The Dursley's were having a wonderful time, not knowing, of course, that by the end of the day, they would be facing investigations of child neglect. They came out of the zoo to find an angry woman standing by their car.
"Ma'am, would you please move away? We are trying to leave, and you're fairly close to our car." Vernon Dursley said, keeping up a mask of politeness, though he was quite annoyed with the woman.
"Oh, this is your car, then? No. I won't be leaving. I would like to know, though, who is the two-year-old?" The Dursley's paled, realizing what the woman had seen and how it would be perceived. Dudley Dursley, who had no idea of the problem, decided to try to help the lady.
"Do you mean Harry? He's my cousin." This information rewarded Dudley with a strained smile.
"Thank you, sweetheart. I wanted to know why he was in the car. Do you know?"
"Daddy says Harry's a freak. I'm not sure why." Dudley hadn't noticed his parents' attempts to quiet the boy, and he liked the lady. She seemed very nice to him.
Maria grew angrier at the small boy's words, though she smiled at him and thanked him. She suggested he go play with Luke while the adults talked. After a quick look at his parents, Dudley agreed. It wasn't long after that a child protective service worker arrived. By this time Harry had woken up, and he had joined the other three children in playing. Dudley seemed a little reluctant to play with his cousin at first, unused to him as a playmate, but soon the four children were playing happily, an emotion quite different to what the adults were feeling.
Jennifer Kersten was very new to the job of protective services, and so, she was more upset by the story presented to her than one of her more experienced colleagues might have been. It was decided that she would go to investigate the child's home life the next day, seeing as she had no other work that day.
Before she left, Maria Duncan had asked if she could keep her updated on the young boy. Jennifer hesitated, not certain of the rules in this case, but fairly sure that she wasn't allowed to tell. Maria gave her the family's phone number, just in case.
The next day, Jennifer arrived at number four Privet Drive. Harry had been instructed to inform her that his was the smallest bedroom, the one full of Dudley's old and broken things.
When Jennifer was shown the room, she soon noticed the state of everything there. The Dursley's had cleaned the room somewhat, but it still was not a proper bedroom.
"Harry, why is everything here broken?"
"Uncle Vernon and Aunt Petunia put all of Dudley's old things here."
"Here? In your bedroom?" Harry squirmed a bit as he replied positively.
"Harry, is this really your room?" Harry shook his head.
"Will you show me your room?" Harry nodded before heading downstairs, and showing Jenny the cupboard where he slept. He explained the chores he did. There weren't many of them, he was only three, but he shouldn't have any chores.
In the end, it didn't matter that Jennifer hadn't told Maria what happened with Harry. There was a small article in the local newspaper about the three-year-old who was taken away from his guardians.
Maria pleaded with her husband to go adopt the young boy they had seen at the zoo. He protested a bit, but he cared for the child too, and found that he couldn't quite refuse. Before the year was out, Harry Potter had become a member of the Duncan family.