Adaptation Induced Plothole: In his fight with Alucard, Luke is shown to have a strong Healing Factor, surviving a head shot. However he is unable to regenerate his legs. In the manga this inconsistency could be explained by the fact that Casull inflicted the head shot, while his legs were shot off by Jackal. In the OVA, Jackal was the cause of all three wounds.
For those of you who do not know the names: he used a MUCH better gun for the leg shots in the manga, and a more standard one for the headshot.
Luke simply ran out of regeneration fuel. Alucard has hundreds of thousand souls to get life force from, hence the Healing Factor. Luke is a young vampire and probably hasn't killed a lot of humans yet. On top of that, he's an artificial vampire, and they all suck.
There was no headshot in the manga; he shot him in the stomach with the Casull, just like in the Anime. As for the wounds themselves, Seras' injury in the 7th OVA/Volume shows it's much easier to heal a bullet wound than a fully-severed limb, which may be the cause.
Rectified in the Blu-Ray rerelease, where Luke's initial injuries are now caused by the Casull.
In the 8th Volume of the manga, Alucard, in trying to kill Anderplant, never thinks to shoot him in the heart; in the OVA, he does. The problem: Anderplant and the nail regenerate the wound. This creates a Plot Hole because the next Volume/OVA features Alucard ripping his heart out by hand, something which won't make sense in the OVA because it would appear to be a regenerable wound.
Not necessarily. The regeneration power may be enough to fix a hole in the organ, but not enough to replace an organ that has been completely ripped out. Especially if the nail remains attached to the heart, rather than Anderson himself.
If you look at the bullet in the OVA, it is evident that it hit the nail, and is stopped by it, spinning in place and not hurting the heart itself.
Girlycard, Alucard's young girl form. Also known as Lolicard in the Japanese fandom.
Anderplant for Anderson's monstrous form after he stabs himself with Helena's Nail.
Since the Major is never actually named, several names for him have popped up, Montana Max being the most popular.
Taliesin Jaffe, the director for Hellsing and Ultimate's dubs, calls the Major this since he's a fan of the manga.
Hans Günsche and Avondale Napyeer, fannames for Captain and Doc, respectively.
Montana Max and Hans Günsche come from the prototypes of the characters in older works. On the back of Volume 5, you can see a small drawing of some papers with Major's face on it with some reading "Montana Max".
Jossed: Taliesin Jaffe, the director of the dubs of the TV series and Ultimate, held onto the theory that the Major is actually Mars, the god of war (which would explain his motive). Said jossing was when Hirano showed him to actually be a cyborg.
The Other Darrin: Zorin Blitz was the only character to receive a change in voice actors. Helena Taylor voiced her in OVA 4, but Taylor had to go back to England during the hiatus of the dubbing, so Rachel Robinson filled in for her for OVAs 5-7.
Relationship Voice Actor: On the less well-known side of things, JB Blanc, Victoria Harwood and Steven Brand — the dub voice actors for Maxwell, Integra and Anderson respectively — have known each other since the 80s. According to the DVD Commentary for OVA 3, during the casting for the original anime series, Steven Brand brought Victoria in, and Victoria brought JB.
Schedule Slip: 10 years to finish a 10 volume manga, Hirano Kohta is (in)famously known for being quite a lazy mangaka when editors are not keeping constant watch on him, near the end of the series it seemed Hirano would release a new chapter whenever he felt like it, he was often taking breaks from working in a monthly magazine and also some of the near end chapters are just 9 to 12 pages long!
The OVAs also followed suit in what it seems to be a staple in the franchise, the first three episodes had just a few months between each other, from the 4th and on the delays would just get longer and longer as Satelight was having a hard to time to keep producing the episodes all by themselves, after a year and half without releasing anything Satelight passed the baton to Madhouse and finally looked like they would keep up the pace, and they did... only up to the 7th episode, another year and few months (and many rumors that Madhouse just dropped the series) is what it took for the 8th episode to get a release date; the series finally finished with the release of the 10th episode in late December 2012, nearly seven years after the series began.
And the official US releases were worse, with the original license holder going bankrupt, the gap between releases was 4 years, with two Blu-Ray/DVD sets compiling the eight OVAs so far finally being released in October and November 2012 after the release of the DVD for Episode 4 in September 2008.
Hirano topped himself with the prequel, Hellsing: The Dawn, after 6 chapters he simply stopped in 2007. The Dawn didn't even have enough chapters to publish a single volume, so the series remains just a collection of 6 separate publications in 6 different Young King Ours+ issues.
Series Hiatus: Hellsing: The Dawn. Hirano felt like stopping out of nowhere in 2007, and yet Young King Comics did not declare the series as finished nor cancelled.