I have seen some confusion over the pinouts of the Mega2560 interrupts, well I'm confused about some of the documentation.
This site shows digital pin 21 = INT0 Mega2560
This site shows digital pin 2 = INT0 Mega2560
https://www.arduino.cc/en/Main/arduinoBoardMega
This site shows digital pin 2 = INT0 Mega2560
Am I seeing this incorrectly?