How Did Professor Slughorn's Mead Get Poisoned?

In Harry Potter and the Half Blood Prince, Ron has the great misfortune to be nailed with a nasty love potion. Although Ron's undying love for Romilda Vane is quite amusing, Harry decides to take him to Professor Slughorn to get an antitode.

Sure enough, the potions master brews up an antidote and saves Ron from further embarrassment. To celebrate, Professor Slughorn offers the boys something to drink. He eventually picks out a bottle of Madam Rosmerta's oak-matured mead. To everyone's surprise, it is poisoned and Ron nearly dies before Harry thinks to stuff a bezoar into his mouth as an atidote.

But how did the mead get poisoned in the first place? This, it turns out, is a slight difference between the novel and the movie.

The Novel...

When Slughorn suggests that they drink the mead, he mentions that it was originally intended to be a Christmas present for Dumbledore. Christmas had come and gone, though, so apparently he failed to deliver the present. The implication is that someone tampered with the bottle of mead in an attempt to poison Dumbledore.

As the book draws to a close, Malfoy lets loose the secret. He placed Madam Rosmerta under the Imperius Curse and pulled her into the plot to assassinate Dumbledore. That's how he delivered the cursed necklace to Katie Bell. It's also how Malfoy found out that Dumbledore left Hogwarts when he and Harry had gone in search of the horcrux.

After Malfoy indicates that Madam Rosmerta was acting as his accomplice, Dumbledore surmises that she poisoned the bottle of mead before it was delivered to Professor Slughorn.

The Movie...

Initially, the story develops in a similar fashion. Slughorn doesn't mention who the mead was intended for when he opens the bottle, but in the next scene in the hospital wing he actually tells Dumbledore about this bit of trivia. In the book, it's unclear whether or not Dumbledore knows (at the time) who the mead was intended to be given to.

As the story draws to a close, though, a significant difference appears. Malfoy confesses his plan, but Madam Rosmerta is conspicuously absent. She was never Imperiused, and she never helped him.

Dumbledore still surmises that Draco Malfoy was behind the poisoning. Neither of them spell it out, but the implication is that Malfoy tampered with the bottle of mead himself to deliver the poison. Since he makes no mention of a cursed accomplice, it makes more sense that he did it himself than that the bottle was poisoned at the time it was given to Slughorn.

This is another example of a minor detail that was changed in the movie as they attempted to streamline the plot and simplify the story's progression. Although Madam Rosmerta's involvement was a nice little twist, the directors must have deemed it unnecessary. She unfortunately plays no role in the plot, and indeeded there is not even a listing of Madam Rosmerta on the IMDB credits for Harry Potter and the Half Blood Prince.

What a shame.