Pin 2 has an interrupt when shift to HIGH to LOW.
If I use a button to shift HIGH to LOW, or specially I connect a wire from Pin 2 into ground line... the arduino hangs (many times).
Show us your code? Did you use FALLING or LOW interrupt?
That way a new interrupt does not interrupt the ongoing interrupt, so to speak.
An interrupt does not fire while an interrupt is being processed, but certainly they may happen in quick succession if you don't debounce.
Another issue is that he might be trying to do too much in the ISR.
Posting code gives better answers.