In the Laundromat scene when Jamie and Russ are talking you can't see outside through the glass but you can see the ceiling lights reflected in the glass (As in darkness/nighttime). Although it was daylight when they left Mark to try to pick up chicks in the previous scene.
The Countess's dress changes from a shoulderless one when she first meets Mark in the club, to a short-sleeved dress upon exiting the club and removing her jacket at home. In actual fact, it's the same dress. In the club, she wore it as a halter-strap plunging neckline dress that appeared to be backless as the material was gathered up under her hair. However, this type if dress allows the wearer to straighten out the material so that it covers the shoulders and back. When Sebastian is helping her with her make-up, you can just see her bare back underneath it.
Mark's reflection in the dressing-room mirror does not match when the Countess's reflection is missing (the lighting and movements are wrong). Also, outside the dance, Robin faces one way while backing away from a reflectionless Mark but her reflection faces differently.
In the costume party/dance scene, when Robin and Countess are having a dance fight over Mark, the camera is zooming in on the dance, to the very left of the screen you can see the lens hood of a second camera zooming in.