Where is a paradox in Harry Potter and the Stone?

In Harry Potter and the Sorcerer's Stone, when Harry, Ron, and Hermione meet Hagrid, he tells them that he needs to get Fluffy past an obstacle course to retrieve the Sorcerer's Stone from the third-floor corridor.

However, when Harry, Ron, and Hermione later go to the third-floor corridor themselves, they find Fluffy guarding a door at the end of it, not an obstacle course.