I did a 6 month internship in the US (with a Visa) and arrived at Philadelphia airport without the letter from my internship place detailing what I was going to do: the US Embassy in The Netherlands had it, I only had one copy of said letter.
After explaining my story, what I was going to do where, how long I was to stay, which loophole I exploited (volunteers for religious non-profits get a B1 visa), who was going to pay (my parents) and what their occupations were, and that the non-profit was not paying me in any way I was free to go. Easy if you ask me: that could've been quite a struggle to get myself out of that one.
After explaining my story, what I was going to do where, how long I was to stay, which loophole I exploited (volunteers for religious non-profits get a B1 visa), who was going to pay (my parents) and what their occupations were, and that the non-profit was not paying me in any way I was free to go. Easy if you ask me: that could've been quite a struggle to get myself out of that one.