- Ability over Appearance: Jim Broadbent's Slughorn is different from how the books describe him - short, incredibly stout and bald, and with a walrus-like moustache - but he displays the shrewd opportunistic yet friendly quality perfectly.
- Billing Displacement: As has become a common trend of the films by now. The veteran actors are billed in the main credits as usual. Most glaringly, Timothy Spall is listed as a major star despite having only six seconds of screen time.
- California Doubling: The only time in the series this happened; the exteriors for the cave (which is Cornwall in the book) were filmed in the West of Ireland.
- Creator Backlash:
- Daniel Radcliffe views this as his weakest performance in the series. He was also having troubles with alcohol at the time, which likely contributed to this.
- In a note that Alan Rickman wrote to himself during the production of this film, and which was released to the public years later, the actor declares frustration with David Yates, believing that the director "has decided that this is not important in the scheme of things i.e. teen audience appeal."
- Hide Your Pregnancy: Helena Bonham Carter (Bellatrix Lestrange) was pretty noticeably pregnant in the film. The scene where she and Narcissa visit Snape has a couple of shots where her belly is very poorly hidden, even wearing a flowy black dress. She spends most of the scene in the background and/or hiding behind the furniture and Snape and Narcissa. It's better hidden in some of the other scenes.
- I Knew It!: Before the release of the book, oddsmakers had Dumbledore as the character most likely to be killed and they were right.
- Orphaned Reference:
- In the attack on the Burrow, Ginny inexplicably does nothing once Fenrir Greyback shows up. As shown in the trailer, he disarmed her from afar. Ginny can be seen picking up her wand as soon as Lupin and Tonks get there too.
- The twins Flora and Hestia Carrow are given a lot of attention during the Slug Club scenes. This is a holdover from an earlier draft when Harry would realise that the Vanishing Cabinet had a second one.
- The Other Darrin:
- Frank Dillane replaces Christian Coulson, who played Voldemort's teenage self in the second movie. This was due to Coulson now being viewed as too old for the part, despite him having been so in the first place (23 playing a 16-year-old). Oddly enough, this recasting carried over to the Japanese dub, wherein Jun Fukuyama replaced Akira Ishida as the character's voice.
- Minor example. Lavender Brown had been played by background extras in previous films. Here when she has an actual role, she's played by Jessie Cave - and given a Race Lift (the previous two actresses were both black).
- Real-Life Relative: Tom Riddle's eleven-year-old self is played by Ralph Fiennes's nephew Hero Fiennes Tiffin. Word Of God is that his resemblance to Ralph and his "dark, haunted quality" played a factor in his casting.
- What Could Have Been: See subpage.
Trivia / Harry Potter and the Half-Blood Prince