Ok, my first plot hole is that very specific kind that I spoke about here:
Basically, the mirror. Not the Mirror of Erised. No. The mirror that Sirius gave Harry in the books.
You see, in the books, Sirius gives Harry a mirror that James used to have. Sirius had one, too, and they could use them to talk to each other. It would have been given to Harry when Sirius presented him with the picture of the original Order of the Phoenix in the film of the same name. Harry completely forgot about the existence of this mirror and it ends up broken some way along the road. However, at the end of the book, Harry finds it and remembers Sirius’s words about using it whenever Harry needed him.
Then there’s this really heartbreaking moment where Harry tries desperately to speak to Sirius after he died. It’s made even more tragic by the fact that Harry literally had the ability to check up on Sirius right there with him and didn’t need Umbridge’s fireplace to confirm that Sirius hadn’t gone to the Ministry. Plus, the thought of Sirius waiting all year for Harry to use the mirror really breaks my heart.
They removed that completely from the film for… reasons? Then, they realise in the Deathly Hallows that they actually need the mirror to make the story work. Harry has a shard of the mirror’s glass in his possession when he’s captured by the death eaters and it turns out that Mundungus has sold Sirius’s one to Aberforth, Dumbledore’s brother, who sends Dobby.
Because they couldn’t think of another way to make Aberforth know they were in trouble, they just had Harry speaking to some random piece of glass we’d never heard about before. For someone who’d read the books before seeing the films (and before being in the final film, cough cough), I just filled in the gaps myself. Recently, though, I realised that people who didn’t read them would have no clue what that shard of glass was about and why Harry thought that anyone would be able to help him by saying “halp” into it.
The creators of the films wrote themselves into a corner and then just busted their way through the wall to escape it XD