Lord of the Rings: Fellowship of the Ring Extended version - Vanishing Ring
In the scene where Frodo tells Galadriel he would give her the ring if she asks for it, she reaches out with her right hand towards the ring. Her hand is bare. A few minutes later, he tells her he feels so alone, and she says, "To be a ring-bearer is to be alone." And reveals to him her right hand, but this time it has her ring on it!
I'm not sure this is a slip-up as such. Professor Tolkien wrote in the book that the three elven rings were kept hidden, and Galadriel chose to reveal Nenya (the ring she wore) to Frodo as he was the ring-bearer. Ordinarily the ring would have remained hidden.