1. I've noticed that it is common in Harry Potter fan fiction (I've been reading some since someone encouraged me to) to have Fred and George Weasley complete each others sentences. While I think it is fine on occasion it gets rather annoying seen too frequently and really isn't the way I remember it from the books. In the books they both did complete sentences just alternating between them on occasion (they were complete sentences not finishing the sentences). The thing is of the fan fics I have read I don't recall seeing Harry and Hermione do so; which I find to be strange since they actually did so on occasion in the books for example in Deathly Hallow p250 of the UK/British edition they did so.
2. In the last book the main trio live out of a tent for awhile and have trouble scavenging food being hungry and eating wild mushrooms. Why didn't they just go to a muggle store and buy some food? I really don't get that. Even if they didn't have any muggle money they could have still robbed someone’s house for food and memory charmed the person since they don't really seem to have moral qualms about altering the memories of muggles what with lots of people doing so during the World Quidditch Cup, common practice to hide magic from muggles, and how Hermione did so to her parents. They needed food and muggles have lots of food which they could have gotten and if they felt guilty for stealing they could have done something in exchange for it like used magic to repair some broken things at the person's house or something. Even if they were trying to avoid using magic they still could have stolen some since they needed it.
3. Fidelius charm. The Fidelius charm is a magic that hides a location. I was wondering about it and can't really figure out how you'd place it on a place with no address. #12 Grimmauld place can easily be written down but small cottage with no address in the woods next to a mountain 23 miles from the nearest road not really easy to write down. I was also wondering if a seer would be able to see the secret or could the secret be taken from someone's mind via the magic that gives access to memories? Also if Dumbledore knew the secret about Harry's parents house (which does seem to be the case since it was stated that he placed a charm on it) how is it he didn't know who the secret keeper was and if he did know why did he let things happen the way they did? I still don't really get why Harry's mom wasn't the secret keeper and just stayed in the house at all times with Harry until it was safe or other plans could be made. It is shown that the secret keeper can live in the place in which they are the keeper of the location.
4. Hogwarts seems to be a pretty crappy school for learning. Beyond just the bad teachers (defense against Dark Arts has consistently had bad teachers). I've seen mention in fics of characters going to a muggle University after graduating. The problem I have with that is that Hogwarts is very much a specialty school on magic and doesn't teach the things needed to be admitted to a university (Science, math, muggle history, literature, etc.). Even the muggle studies class probably is way out of date and just wrong judging by how little people like Mr Weasley know about muggles and his job requires he have knowledge about muggles (Misuse of Muggle Artifacts Office - a job you'd think would go to a muggle born not a pureblood since they have more knowledge on muggles).
5. The Horcruxes trip Harry made with Dumbledore makes no sense to me at all. Seriously none at all. Harry suddenly freezing up when approached by Zombies when he didn't in early books when approached by Dementors which are worse than zombies. Dumbledore somehow came to the conclusion that he could drink a cauldron of something that couldn't be touched. The boat which could only carry one magical person being able to carry Dumbledore and Harry because Harry was underpowered when people were impressed about previous actions which make it very clear that he was quite powerful for example throwing off the imperious curse which almost no one could do (which was part of why it was an unforgivable), his powerful Patronus Charm which impressed a lot of Wizengamot members, etc. Lots of other things don't make much sense either but that seriously annoyed me the stuff that happened afterward makes even less sense the Death Eaters really suck at tactics.
6. I found the whole Elder wand thing to be incredibly stupid and diminishes Dumbledores abilities since it implies that he only became so powerful because he had that wand. The whole defeat someone and disarm them that they can no longer use their own wand because it belongs to them now also makes very little sense.
7. Ron being able to speak parseltongue was, I'm not sure what that was other than terrible. I can suspend my disbelief a lot but not that far. Being able to mimic a magical language so easily even one word is just unbelievable and if it was so damn easy I'm surprised that the Chamber of Secrets hadn't been opened by accident a few hundred times.
8. Why didn't Harry learn more about his family and possibly extended family? He seems to know next to nothing about them when it is something he is clearly shown to want to know and shouldn't have very much trouble finding somethings about them by talking to people who went to school with them if no other way and yet he knows a lot more about Voldemort than any of his own family members. It seemed that only Snape ever really spoke about his parents at all and he was very biased. Why didn't others give him pensive memories or talk to him about his parents or other relatives?
9. The Weasley's are poor. We are told this often in the books and that they tend to be made fun of because of their threadbare clothing. We are shown some pretty damn useless though impressive magic such as turning animals into inanimate objects why can't they use magic for tailoring? Reparo works on Harry's glasses for a long time but there is no spell for fixing clothes? On the subject of clothes. Why didn't anyone take Harry shopping for clothes? We are shown he has money and can afford it (small hills of gold whereas the Weasley's are so poor that the amount shown in their vault is so small that they really have no point in having a vault) and yet we get lines like "'taped shoes, worn, baggy T shirt and torn, oversized jeans" as a description for his clothing in later books. Someone, anyone, should have taken him to go buy clothes (a decent pair of shoes if nothing else) and he should have convinced Ron to let him buy Ron some as well just so that Ron wouldn't be made fun of for his clothes.
10. I've seen fics that use pensives for court cases and other times when showing memories could be helpful. I find this strange since we are shown that it is possible to alter the memories put in a pensive; Slughorn did so. It is possible that Snape or Dumbledore did so and were just much better at altering the memories. I personally found it suspicious that Snape just happened to leave memories in the pensive that show Harry's father in a bad light. A thing to also remember is that memory is mutable and people do and can create false memories. People generally do not remember things the way they actually were just look at eyewitness accounts and see how different they can be and people have a tendency to make themselves look better in their memories. Then there is other things like Polyjuice potion which can create false scenarios. Sure you remember the person doing so but was it actually that person or someone using Polyjuice?.
11. Why do I keep seeing mention of a Weasley temper in fics when Arthur doesn't seem to have much of one and Molly who does have one is only a Weasley via marriage (formally Prewett).
12. The subplot in which Hermione and Ron were made perfects annoyed me greatly and I am glad that it is cut from the movies (or so I heard I haven't watched the movie in which that would be in yet). The reason given way later (near the end) in the book is that Dumbledore thought that Harry had enough responsibility to be going on with. Not making Harry a perfect would have made the students less trusting of Harry since it implies that Dumbledore doesn't trust him. I kind of hate Dumbledore, I don't think he is evil, just pretty damn stupid, insensitive, and a whole bunch of other things. I know the real reason the author did so was to improve Ron and put some conflict in their relationship but I still hate it. I don't think Ron was deserving of the position (much less Draco Malfoy and Pansy Parkinson since they - including Ron - did use their powers as perfects for their own amusement and personal gain) and Harry had every reason to be upset with that. Dumbledore should have given him the position (everybody else had thought so and was surprised Ron got it). Harry could have turned down the position if the author really didn't want him to have it and if it was too much for Harry he could have later given up the position; Dumbledore should have at least asked instead of doing what he did (but Dumbledore is a bit of an asshole practically making Harry's life bad forcing him to live like a prisoner in that he isn't given choices in a lot of things).