Time-turners are a large plot hole, because we already know how to exploit closed time loops/predestiny via Bill and Ted. The use of time-turners to fuck over other wizards is pretty much a given since we know McGonagall warned Hermione from using it this way. The excuse given was
'Professor McGonagall told me what awful things happened when wizards meddle with time...Loads of them ended up killing their past or future selves by mistake!'
Except we know this can't happen because the way time-turners work would prevent someone from killing their past-self (otherwise they couldn't travel back), and if you 'killed' your future-self you could just avoid going back in the first place, which also couldn't happen because of predestiny. All you have to do to effectively use a time-turner is have one and declare that you will use it.

Portkeys are a huge plot hole. They can be touch or time activated, one-way or two-way, and present no danger in use (though Pottermore suggests such a thing as 'portkey sickness'). This being the case, why does anyone use Floo (which can result in missed estinations (CoS)) or apparition (which can lead to splinching/death)? They can't be all that hard to enchant since Order members other than Dumbledore enchant tons of them in DH.

Wordhammer pointed out the Fidelius Charm error for 'Shell Cottage', but how on Earth was the Potter's home in Godrics Hallow visible to Hagrid, and later Hermione and everyone else who left messages there for Harry? In DH it is stated specifically that
'the Fidelius Charm must have dies with James and Lily.'
yet it didn't die with Sirius or Dumbledore?

Ron suddenly revealing his parseltongue ability is a plot hole as well. Or or we supposed to believe that he remembers the exact sounds Harry made 5 years earlier, but has to be reminded every other year about apparition inside of Hogwarts?

The Thieves Downfall in Gringotts is a plothole too. How is it that goblins have this, but the Ministry of Magic doesn't? Oops, guess we'll just be infiltrated and destroyed from the inside again.

 

This is a minor plot hole, but in Philospher's Stone and Chamber of Secrets werewolves are implied to be something different from what they are in the sequels.

There was probably one in Chamber also, but I can't find the quote at the moment.
Thtomitrebo, the thief's downfall plothole could be explained by the ministry not being as paranoid as Gringotts, or it being a recent addition from after the ministry incident.

 

Last time he heard Harry say "Open" in Parseltongue was when they dealt with the locket so I can understand it sticking in his memory. In my mind, I just imagine that he was standing there for a while trying to hiss the same way and got lucky.

 

I was going to dismiss this because, while he grew up in the wizard world, Malfoy could be confused because of his bias towards half-breeds, buuuut in CoS:

 

I'm partial to the notion that you can 'change' things as long as you meet the two criteria that you're not sure you're actually changing them and causality is maintained.

Example: Harry, Ron, and Hermione part ways before she heads to Ancient Runes class. Suddenly, there's a scream. Harry and Ron run down the hall and find Hermione decapitated by a spell.

This is a scenario where most people would say a Time Turner can't fix anything. She's dead, end of story. I say, as long as harry ducks out of there before confirming it's really Hermione, he can change it.

Before it can be confirmed that it's really Hermione and not an illusion, or a transfigured animal, or another person Polyjuiced to look like Hermione, Harry runs off and uses a Time Turner to go an hour or so into the past.

He uses this time to grab a mouse (or a spider, a fuckin' doxy... whatever), transfigures it into a passable Hermione, then goes to an abandoned classroom on Hermione's route to Runes class and lies in wait for her.

When she passes, he stuns her and drags her into the room, shoves the Hermione stand-in into the hallway to be killed (or, in a weird twist, he kills the double himself, meaning it was actually him the whole time - leaving him and Hermione to ponder that fucked up time loop for the rest of their lives). Past!Harry and Ron come running, see what looks like a dead Hermione, and Past!Harry runs off to grab the Time Turner. Since he still has a motive to go into the past and save Hermione, causality is maintained.

Then Future!Harry and Hermione pop back up, safe and sound. All the general public ever needs to know (if anyone else saw the body) is that some undiscovered party was playing a sick joke, or it was an assassination attempt that was discovered in time to be foiled with a transfigured double.

That's just an example, but if you don't think Harry's that good at transfiguration, maybe he uses his extra time to grab some leftover Polyjuice and a junior death eater, then forces them to drink the potion. Faced with the possible death of his best friend, maybe he goes all out and uses the Imperius to get his patsy to drink the potion and then walk out to face the killer. Or, after he grabbed Hermione and explained what happened (or simply uses the Imperius on her because he's on a tight schedule - I'm sure she'll forgive him after seeing 'her' decapitated corpse), she transfigures the 'dummy.'

There aren't too many scenarios where something like this works, and some people may feel it edges too close to the whole "Harry can do so many impossible things with magic because he doesn't know it's impossible" thing that became popular for a while in fan fic, but ignorance of the facts is sort of the key to this working out.

Also, if it's not just the time traveler's causality you have to worry about, you have to perform any substitutions or communications as close to 'zero hour' as possible, to avoid messing with the actions of anyone else.

This works decently for replacing Sirius with a fake, because there's no body to inspect, and there's plenty of time between Harry trying to contact him, and Sirius getting killed. You go back as far as you can/need to, then grab Sirius, explain your situation, and either replace him with some sort of double, as above, or use some very well-coordinated disappearing act with Future!Harry's cloak or a brief illusion, to make it look like Sirius 'vanished' through the veil.

Past!Harry sees him 'die' and the cycle repeats.

Anyway, just a bit of a tangent. Hopefully time travel won't consume the thread. I'm sure it wouldn't stand up to much scrutiny by Hard Sci-Fi (sorry: Speculative Fiction) standards, but for fan fiction of a magical world...

Man, adjusting to a new keyboard is a bitch.

 

Eh. Your scenario requires Harry to not have confirmed that it was Harmione who was dead, but to be so certain that it was her as to go back in time and try to save Hermione specifically from dying. That counts as confirmation, if he himself acknowledges that it happened and needs fixing.

 

It just confirms he has to go back and create the illusion of her death to maintain the loop.

It's not much more of a leap than knowing you can cast a super Patronus because you just realized it was your own self you witnessed, earlier, casting a Patronus (it was super effective).

Time travel is like the afterlife: If you're at all concerned about your day-to-day happiness, it's best to contemplate it as little as you possibly can.

 

Trying to formulate what a time-turner can and can't do logically is a bit of a wasted exercise, since by definition TTs don't actually obey the rules of logic as we know them.

 

Man, Time Travel ALWAYS complicates things. Better to not have it at all.

 

Too true. Or if you do have it, restrict its use to NPCs and never allow its use by protagonists (I'm looking at Butcher).

Reading back over DH there is another plot hole, related to the Trace. The 'Decree for the Reasonable Restriction of Underage Sorcery' was enacted in 1875, yet in the summer of 1943 when Voldemort was 16 he cast three killing curses and at least one memory charm. From DH

'In the summer of his sixteenth year, he left the orphanage to which he returned annually and set off to find his Gaunt relatives.'

He killed his father and grandparents, and placed a memory charm on Morfin. Yet Voldemort didn't even receive a letter telling him to not cast magic outside of Hogwarts.

 

From what I recall of the Trace - and this may be fanon - it's linked to the wand, not the wizard/witch. Riddle used Morfin's wand to kill his father. Doesn't explain Morfin himself though, unless there's another facet I'm forgetting.

 

I think Riddle was devising ways to dispel/trick the Trace from the moment he learned of it. It was probably even before he come to Hogwarts. I'd be disappointed in him if he had not managed this in time to his OWLS.

 

Pottermore actually explains this:

 

No explanation that I know of, but does stupidity really count as a plot hole? People do stupid shit all the time.

 

Maybe they just didn't have that close a relationship with Dumbledore. After all, he was the leader of the Order of the Phoenix and their old headmaster. He'd be trusted and respected, but when the life of your only child is on the line, who are you going to trust more? Your old headmaster or your best friends?

Plus, the entire idea behind the plan was for Peter to slink off and hide while Sirius presented a big target to the world. Nobody was even supposed to know that Peter was the Secret Keeper, which should have made it even more secure than it could be with Dumbledore.

 

If you're going for security through obscurity, leaving the country would probably work much better. It's not like the Potters were poor, or anything.

 

The Wizarding World is a pretty small place though, so chances are if you're well known in one country you will be in another. Plus, I don't doubt that Voldemort could have tracked them down if they just relied on mundane means of hiding.

As it is the plan would have worked if they hadn't picked the single worst person they possibly could have to be their Secret Keeper. Sirius was good enough to avoid an entire government division tasked with finding him, even before the lead investigator stopped trying, so he'd probably be good enough to hide from the Death Eaters as well. Hell, that likely was the plan; Sirius and Peter head off in different directions, leaving the country if need be, with Sirius being the more public distraction to allow Peter to disappear.

Wouldn't have been a bad plan, all told, if Peter hadn't been a traitor.

 

Obviously Dumbledore knows a lot of things. He is their leader and it does make him a natural target. Maybe the Potters were trying to avoid the "put all your eggs in one basket" thing. In case Dumbledore is captured and somehow forced to tell secrets, at least the Potters are safe.