Scarlett Johansson's on set knockout

Robert Downey Jr accidentally punches actress on set of The Avengers

The actor, who plays Toni Stark/Iron Man in the film, was playing with his high-tech costume as he was ready to shoot a scene with Johansson.

The actress, who plays Natasha Romanoff/Black Widow, accidentally fell over when one of the mechanical arms attached to Downey's custome pulled a punch.

The star was hit in the face, staggered backwards and then fell off the low platform she was standing on.

Downey Jr was mortified, but Johansson wasn't hurt and she smiled the incident off.