Director Mike Newell was not aware that Alan Rickman wore black contact lenses for the role of Snape until one day when he was complimenting him on the amazing shade of his eyes. Rickman leaned over and popped one of the lenses out.
In one of the first takes of Hermione's "Cinderella moment", Emma Watson actually tripped in that fancy dress and fell down the stairs.
The underwater scenes were shot in a huge purpose-built tank with a bluescreen background. Safety divers swam in-between takes with scuba regulators, to allow the actors and actress to breathe without having to surface. Daniel Radcliffe alone logged around forty-one hours and thirty-eight minutes underwater during the course of filming. At one point, during training, he inadvertently signalled that he was drowning, sending the crew into a huge panic to bring him back up to surface.
At least one full-scale dragon was constructed on the set, which could even blow real fire. The dragon was created partially from the basilisk puppet seen in Harry Potter and the Chamber of Secrets (2002).
In the movie, the audience is given the impression that Beauxbatons is an all-girls academy, whereas Durmstrang is an all-boys one. In the book, however, both schools are co-ed, and in fact, in the book, the Patil twins leave Harry and Ron to spend time with boys from Beauxbatons.