<p>Downey Jr was wearing his heavy superhero suit while he was waiting to film scenes with Johansson.<br /><br />"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," femalefirst.co.uk quoted a source as saying.</p>
<p>Downey Jr was wearing his heavy superhero suit while he was waiting to film scenes with Johansson.<br /><br />"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," femalefirst.co.uk quoted a source as saying.</p>