The actor was wearing his heavy superhero suit while he was waiting to film scenes with his glamorous co-star in their new movie 'The Avengers' and was fiddling with the high-tech gadgets attached to his outfit.
However, he suddenly hit the 'wrong' button which controls one of the arms and hit Scarlett - who plays Black Widow in the movie - in the face.
A source said: "Robert triggered a switch that controls one of his arms - and it suddenly jerked backward and whacked poor Scarlett. Knocked silly, she staggered backward, then lost her balance and fell off the low platform they were standing on."
Stunned crew rushed to help Scarlett get to her feet and luckily so was unharmed and saw the funny side.
The source added to National Enquirer magazine: "Robert, who couldn't help because he literally can't move in that iron suit, was beside himself - he kept apologising and saying, Oh, my God, I'm so sorry!' But Scarlett was a good sport and smiled about the incident."