Flight controllers instruct pilots to dial a certain code in the transponder to identify them from other traffic. The code varies from plane to plane and controller to controller. At least in private planes, pilots are trained to dial the last two numbers first then the first two numbers in order to avoid accidentally dialing one of the emergency codes. So, in your example, a controller could give the pilot a 7501 transponder code. The pilot would dial the 01 first to avoid the possibility of accidentally dialing 7500.
Thanks, doesn't sound so much like a big boo-boo then as much as a false alarm? Hopefully.
If I'm not mistaken, no controller will assign you a 77XX, 76XX or 75XX-series code. The first two digits are reserved for the appropriate emergency. Therefore if you accidentally put in 7521 when you were assigned, for example, 7021, the bells would go off and you would be challenged.