The Tv stars were in the Italian city to represent fashion house Miu Miu and its short film series Women's Tales at the festival, and they were posing on the red carpet when the cardboard backdrop fell down and hit them on the head.
Mara joked about the incident on Twitter.com, posting an image of her and the Girls actress giggling as men rushed to restore the poster to its original position.
She writes, "Best red carpet ever w/ (with) @lenadunham @MIUMIUofficial when a poster fell on our heads while we tried to be sultry..." and Dunham adds, "Unlike most things, this was truly Shocking."
Kirsten Dunst, Dakota Fanning and British actress Felicity Jones were also at the event.
The film festival runs until 6 September (14).
This overlong comedy is so episodic that watching it is exactly like sitting through five...