If I understand this... the pin is usually used say with a Sensor triggering an Interrupt when the device is operating (say counting pulses from a weather-vane)... but when in SETUP Mode you also have a Button on the same pin for user entry (presetting Alarms and things)... In that kind of scenario, absolutely no problem. The only thing you have to worry about is somebody inadvertantly pressing the button during normal operation and potentially screwing that up by triggering your interrupt.