The Rust & Bone star flashed her thighs through a sheer panel in the yellow frock.
Cotillard wore a yellow Christian Dior gown - teamed with a silver necklace by Chopard - to Sunday's (February 10) awards ceremony at the Royal Opera house.
Jennifer Lawrence was also wearing a Dior dress last month when she had a similar leg-flashing incident at last month's Screen Actors Guild Awards.
Lawrence's dress appeared to separate as she walked up to the stage to accept an award for her performance in Silver Linings Playbook. The fashion house had to clarify initial reports that her dress had torn.
Argo was the major winner at the EE BAFTA Film Awards, taking home three trophies including 'Best Film' and 'Best Director' for Ben Affleck.
Watch Digital Spy chat to the stars at the BAFTAs below:
EE BAFTA Film Awards 2013 - red carpet arrivals - photo gallery:
Copyright: David Fisher/Rex Features