Tony Stark: What's the vibranium for?
Ultron: I'm glad you asked that, because I wanted to take this time to explain my evil plan.
When Captain America is fighting Ultron in the truck, for a brief moment you can see a safety line holding actor Chris Evans as he's hanging on the side of the cab of the truck.
When all the Avengers are attempting to lift Thor's hammer, Captain America is able to move it slightly. In the comics, Cap is one of the few characters noble enough to be able to lift Thor's hammer (notable others being Storm and Beta Ray Bill).