At the party when Jim is walking in slow motion towards Nadia you see an older man with a blue shirt and clipboard walk behind him. It's really noticeable
I somehow doubt that some guy would be stupid enough to walk across set while they were filming. Every behind the camera shuts up and pays attention as soon as they hear 'action!'. You are either seeing wrong, or the guy was meant to be there.
I looked watched the movie and you say an "older man" walks behind Jim with a clipboard....... he isnt an "older" man..he looks the same age and everyone else there.I dont think it was a slip-up!He was just one of the guys at the party.