The film script of the final Harry Potter book The Deathly Hallows was found abandoned in a pub after the movie's crew went on a major drinking session.
The 118-page script was handed over to The Sun by a reader, reported the website of the British tabloid.
The script - marked "Private and Confidential" - was spotted under a table and revealed dramatic changes from novel The Deathly Hallows.
As soon as the lost script was returned to production house Warner Bros, it launched an urgent investigation into how the inch-thick booklet was lost.
The $380 million film, starring Daniel Radcliffe as the schoolboy wizard, is currently being completed at Leavesden Studios in Kings Langley, Herts. A large group of film crew went to a nearby pub, said reports.
"They'd got behind schedule in filming but then caught up, so they went out for a celebratory drink which led to many more," said a source from the pub.
The Sun reader who found the script said: "I couldn't believe my eyes. I know guys that work on the film drink in the pub, but it's very embarrassing that someone could just leave it there. I'm glad I've helped to stop any surprises being leaked."
The final Harry Potter book has been split into two films. The first film - with Daniel's co-stars Emma Watson and Rupert Grint back in their usual roles - is due for release in November and the second part will follow around June 2011.