Okay. So I was rewatching OotP ... again. And I found a lot of mistakes in the film. These mistakes are ONLY film mistakes and are NOT differences between the movie and the book. Here's what I found so far:

• During the Occlumency lessons with Snape, Harry's sweat stains on his jumper change in size.

• When Seamus apologizes to Harry in the Great Hall, the placement of the things on the table change.

• As Luna posts a sign for her lost belongings, a shot of her sneakers are shown dangling above her head. Her voice is then heard but her mouth isn't moving.

• When signing up for the DA, the members use a pencil, not a quill.

• During the scene where Bellatrix Lestrange comes out behind Lucius Malfoy in the Department of Mysteries, her voice is heard, but her mouth is not moving till the next shot.

• When Harry and Seamus are yelling at each other in the Gryffindor Common Room, Seamus's shirt is hanging out the bottom of his jumper to begin with, then in the next shot, it's half tucked in and he hasn't touched it!

• In the scene where DA group are writing lines with Umbridge, one of the twins is shown dipping his quill into an ink pot and a few seconds later in the scene the ink pot is not there.

• In the scene when Harry has to write "I must not tell lies" Harry's left hand is lying on the desk, so that it can be seen clearly. The cameras then change and show Harry from Umbridge's point of view and both of his hands are under the desk, then the cameras zoom back and Harry's hand is back on the table.

• When Harry sees Hermione and Ron for the first time in the bedroom at headquarters, Hermione's hair is down and it keeps changing sides. It's hanging down on one side and then in the next shot it's on the other.

• In the scene where Bellatrix and Lucius arrive in the Room of Prophecies and Bellatrix is standing behind Lucius with the DA standing opposite them, she says something but her lips are not moving.

• In the scene where the trio leave hogwarts to go home, the train says "Hogwarts Railways" instead of the noramal "Hogwarts Express".

• In her office, when Umbridge adds sugar to her tea, she places the spoon on the saucer. When Harry walks in and she says, "You're going to be doing some lines today," there is a close-up of the tea cup and the spoon is gone. It reappears in the next shot.

• When Harry first dreams about the Department of Mysteries there is a shot of him wearing a light blue striped t-shirt. They then show him sit up with Ron watching him. However, when they cut back his shirt has changed and he's now wearing a dark blue long sleeved shirt (the same one he wears when he dreams of Mr. Weasley getting attacked!)

• When Harry, Ron and Hermione are talking with Hagrid about the giants, Harry's left hand is lying on the table next to his mug clearly unscarred, even though this scene takes place after his detention with Umbridge.

• In the scene at Harry's hearing he's sitting in a chair but when Dumbledore is talking to Fudge we get a close-up of his his face and in the background Harry is sitting on the benches above the chair, though Harry never moved.

• In the scene where Harry and Mr. Weasley are at the train station, Harry reaches in his pocket for the tickets. In the next shot he reaches again, and then again in the next one. That is a total of him reaching and taking the tickets out of his pocket three times.

• In the scene where Dudley and Harry are in the tunnels when the dementors attack, Harry yells ''Expecto Patronum'' He is pointing his wand at the dementor then gets up and he is not pointing his wand at the dementor. It flicks over to another shot and Harry is pointing it at the dementor.

• In the scene where Harry, Hermione and Ron meet Grawp for the first time, Grawp hands Hermione a pair of bicycle handlebars. She rings the bell. In the next scene where Hagrid is explaining that he is the only family Grawp has, Grawp has the handlebars back without Hermione handing them back to him.

• When Harry, Ron, Hermione, Fred, George, Ginny, and Neville are walking back to Hogwarts from the Shrieking Shack and their walking on the bridge, they show them walking and in the background is a mountain, well they continue walking for a good 10 seconds and the background changes and they're still in the same spot!

• Before Harry walks into detention, Professor Umbridge adjusts her crooked pen. But when it cuts to Harry walking in the door, the pen is crooked again.

If you found any other film mistakes, post here!

That's quite a lot. I was watching OOTP a couple days ago, and found 1 mistake:

After the scene when Harry got that vision of Sirius being tortured, Harry, Hermione and Ron are walking up the stairs wearing robes. The next scene in Umbridge office they're wearing muggle clothes. Unless they went to the Gryffindor tower first, but as seen that they were in a hurry...

Yeah. I noticed that too. But I think in the book they changed into normal clothes or something. I need to read up on that to make sure. I dunno. I suppose it's the whole thing with squeezing everything into a timeframe.

But yeah. That's another thing!

Well spotted!
The films do seem to contain a lot of continuity errors - but that is expected on that sort of set. There are lots of minor details that are moved around (like the inkpot, as you rightly said).
A set with hundreds of kids on it must be quite a messy one, I presume

This isn't necesarilly a mistake, but it's very difficult to get your head round.

In the department of Mysteries, you hear Luna use the 'Levicorpus' spell. Now, I would love to know how she found that spell
Maybe that means the sectumsempra and levicorpus bits may not be in Half-blood prince?

In my opinion, a big mistake!

Let me know what you think

Wow, I never noticed any of these, except for the Levicorpus part. That was intriguing to me.

Yeah, i agree.
I think its a huge mistake too.

Isn't there a shot in the RoR earlier where she's perfoming the same spell (levicorpus)? I thought they learned the spell in one of the DA meetings (in the film).