During the Fantastic Four's first public "outing" on the bridge, the others try to get to Ben, who's kept in place by police officers.
Unfortunately they obviously can't get past some other officers who try to keep passers-by away from the accident.
Reed's solution: Sue has to become invisible, put off her clothes and get past the police to get to Ben.
After some public exhibitionistic scenes she finally manages to disappear, completes undressing and reaches Ben. Just seconds later Reed joins her, obviously without any issues.
How did he manage that? Wasn't "stealth" the only option? At least the two officers pinning down Ben got distracted by the electric sparks and him running away, but what about the others?
I've seen the movie several times so far, but this part always looked forced to me just for the plot (and possibly some fan service).