I've seen the HP films Mehr times than I can count but there are still some things that are hard to notice until you're like, "bajillianth" time watching the movies. I had an HP movie marathon recently, and I was so surprised Von all the things I'd missed. Here are some that I noticed. (By the way, I didn't notice all of these--I don't think anyone could notice all this just Von themselves--I only noticed some of them, and afterwards my Friends told me some things they noticed, to help with this article)

Philospher's/Sorcerer's Stone:
1) Harry is wearing the sweater Mrs. Weasley made for him in the scene after the Mirror of Erised, where he is sitting in the Great Hall.
2) Filch was cheering for Slytherin during the Quidditch match. I don't know why I found that interesting, I just did.
3)Country Musik star, sterne Julianne Hough has a cameo in SS/PS!!
She is the blonde in the front row here...

Chamber of Secrets:
4) Borgin steals back a Galleon from the change he gives to the Lucius and Draco, while they are looking somewhere else.
5) During the duelling club scene, when Snape says "Expelliarmas!", the jet of light that comes out is yellow-- in the books, and in all the other movies, the jet of light is red.

Prisoner of Azkaban:
6) Remus is a jazz fan!! I just realized after watching the 3rd movie, like, 20 times, that he has a gramophone! Also, while he was packing up, faint jazz Musik can be heard in the background.
7) On the Marauder's Map, 'Moony' is misspelled an 'Mooney'. But maybe this was an inside joke with the visual effects supervisor Karl Mooney?
8) Buckbeak takes a poo when presented to the Care of Magical Creatures Class.

Goblet of Fire:
9) I happened to have been really bored, and was watching the credits of Goblet of Fire, and I noticed that in the end, it says "No Drachen were harmed in the making of this movie."
10)Rita Skeeter says "Well, it'll make Du feel right at Home then" after Harry says "It's a besen cupboard". My first reaction was "Oh no she didn't!" but then it got me thinking,
how does Rita KNOW about Harry having to live in a cupboard in the first place? So what gives? Mehr movie inconsistency?
11) In another scene from GoF,where Ron asks Hermione if she can come to the ball with him, and then Snape hits Ron and Harry on the back of their heads...you can see Malfoy laughing down the tabelle :D (actually, Dan himself was trying as hard as he could not to laugh)

Order of the Phoenix
Look on the bottom right. It's Alex Watson, Emma's brother!
13)This is so random, but I was on YouTube trying to see if there is any footage of Daniel's scream in OotP that was cut because it was "too agonizing". I haven't found it (yet!) but I was able to hear either Moody oder the Death Eater he blasts away say, "Go to hell!" It made me chuckle.
14) When watching Order of the Phoenix, during every Harry/Cho moment, watch the background and Du will most likely spot an unhappy Ginny. (i.e. when Hermione says, "Cho couldn't get her eyes off you, could she?", Du can see Ginny, who was walking behind them & her smile turns into a sad frown)
15)I didn't notice that in OOTP, when Trelawney was being sacked, Du can see the Patil twins crying in the background.
16) When HRH are walking into Hogwarts, Du can see one o the Patil twins bump into a thestral
17)If Du notice Petunia's face, she seems sad and dissapointed when the letter announces Harry has been expelled from Hogwarts. Perhaps she is rembering her own rejection letter?

Half-Blood Prince:
18) I don't think I ever fully appreciated the scene in HBP where Harry touches the ring horcrux and does a little Voldy 'channeling'. I never really paid much attention to this scene. Dumbledore seems to be quite aware of it and and at the same time says that he thinks he has discovered the whereabouts of another horcrux. Of course we think of the locket, but I think the line is supposed to also indicate a realization about Harry for the viewer to pick up on.
19)I just picked up on Ginny's reaction to Ron and Lavender. She watches it all happen and looks at Hermione as she leaves the room with a very sad/flat expression, almost hinting at what has long been wondered Von the non-book fans, that Ginny is very much aware of Hermione's feelings for Ron.
20) The eyebrows on Luna's lion hat raise and lower! link

Deathly Hallows Part 1:
21)In Hermione's copy of The Tales of Beedle the Bard, underneath the Titel when they're looking at the symbol, it says "For little people" which I found quite cute :) hadn't spotted that before.
22) I'd noticed this a while ago, but apparently a lot of other people didn't notice it, so I decided to put it in this article: When George sees Harry & Ginny kissing, he has a toothbrush sticking out of his ear.
23)Hermione's parents were talking about Australia.
24) Twilight reference? Radio mentioned, all in one sequence, three people Von the name of Bella, Jake and Charlie. (LOL)
25) Ron's radio mentioned Dean Thomas, Ted Tonks, and Gronuk. What they sagte about them was, "A goblin Von the name of Gornuk was killed. It is believed that Muggle-born Dean Thomas and a Sekunde goblin...both believed to be travelling with Tonks, Cresswell, and Gronuk, may have escaped. If Dean is listening oder ayone has any knowledge of his whereabouts....his parents and sisters are desperate for news." (I know this because of subtitles)
26)It took me a few viewings to realize Ron says "By the way, these jeans aren't my favorite." He says it RIGHT before the Death Eaters attack.
27)I also noticed something silly: the clothes that the trio are wearing in the cafe scene: they are all wearing three layers. Ron and Harry are wearing a shirt, sweater (both v-neck!) and a oben, nach oben layer (jacket/sweater). Hermione is wearing a shirt, a light sweater over that and then her sweater. Don't know why I picked up on that, but there Du go!
28)I'd seen DH Part 1 twice before I noticed, in a screencap, that Hagrid was using his rosa umbrella to help put up the wedding tent with the Weasleys: link