She was confused by US Passport control as an unaccompanied, minor boy. She even goes so far as to note that it's not the first time it's happened.
http://www.upi.com/blog/2013/01/07/JFK-airport-confuses-Emma-Watson-for-underage-child/4011357575149/?spt=fsb&or=ros
http://www.upi.com/blog/2013/01/07/JFK-airport-confuses-Emma-Watson-for-underage-child/4011357575149/?spt=fsb&or=ros