CrazyVito11:
I am creating a sketch that needs to be completely fail-proof
It might be possible to create such a sketch, but of course you could never know if its 'completely fail-proof' as you would need an infinite amout of time to test it.
I suspect you really mean a sketch that is fairly reliable.
Using the processor internal watchdog can help, but if something goes wrong (with the code) before that is setup the internal watchdog is not going to work as required.
An additional independant, timer based watchdog, such as the MAX6369 makes a lot of sense in such applications.