Continuity mistake: When Vivian is eating dinner and the snail "flies" away from her, it goes to the far right of the screen disappearing. Then in the next shot the waiter catches it, but it flew too far to the right for him to catch it.
Continuity mistake: When Edward and Vivian kiss for the first time, her white nightgown goes from being long to being short.
Continuity mistake: In the screen where Vivian has been on the shopping spree, she returns to the hotel wearing the cream colored dress and a black hat. When she walks into the hotel, her black purse is under one arm. Then they show a behind shot of her walking away and the purse is under the other arm.
Continuity mistake: At the beginning when Vivian is sitting on his desk she says " you can pay me now" and he hands her her money and she puts it inside her boot. Later, after the Champagne comes and she is sitting on the steps in the hotel room she asks "do you mind if I take off my boots." When she does there is no money, not even in the knee highs she is wearing under her boots.
Chosen answer: Posters are usually designed long after the film has been made, and by the marketing department. Decisions about the outfit may have been based solely on what was considered best by the marketing department (for instance, to match the color of the lettering in the title.) Also, Julia Roberts' head has been super-imposed onto a model's body in the poster image (just as Richard Gere's hair has been darkened). These are all creative marketing decisions.