during the scene when Kate and Leopold are having dinner on the rooftop. if you look at her right dress strap you'll see it has slipped down a bit exposing her black bra strap. in the very next scene, when she stands and starts gathering plates, you'll see that the dress strap has been placed back over her bra strap.

it's quick, so you'll have to look hard.

just a little slip-up, but a slip-up none the less.