That is just plain false. The rules are not as clearly defined in HP (because it's not trying to be a hard magic system), but when Rowling makes a rule she follows it. For example, you can't bring people back from the dead. That's a rule which is clearly stated and is followed through the series.
In your example, no rule was broken. The book never said "things can't fly in this world". In fact, it's established pretty early on that broomsticks can fly by means of specific enchantment spells. The fact that someone got the idea to apply those enchantments to a car doesn't break the rules of the world. The fact that Harry's friends showed up to rescue him at the right moment is certainly convenient, but also doesn't break the rules of the world. The book explains why they were worried and came to check on him, after all. No rules were broken in this scene, you just don't like it. Which is certainly your right! But it's not poor writing which breaks the rules of the world.
That is just plain false. The rules are not as clearly defined in HP (because it's not trying to be a hard magic system), but when Rowling makes a rule she follows it. For example, you can't bring people back from the dead. That's a rule which is clearly stated and is followed through the series.
In your example, no rule was broken. The book never said "things can't fly in this world". In fact, it's established pretty early on that broomsticks can fly by means of specific enchantment spells. The fact that someone got the idea to apply those enchantments to a car doesn't break the rules of the world. The fact that Harry's friends showed up to rescue him at the right moment is certainly convenient, but also doesn't break the rules of the world. The book explains why they were worried and came to check on him, after all. No rules were broken in this scene, you just don't like it. Which is certainly your right! But it's not poor writing which breaks the rules of the world.