How so? I don't see the connection to undeciability.

If you have undefined behaviour after an infinite loop that breaks on some condition, then that undefined behaviour will not necessarily have been triggered. Thus, no warning.

Regehr himself "disproved" Fermat's Last Theorem by abusing a non-terminating loop and over-aggressive compilers.

http://blog.regehr.org/archives/140

Yes, this I a great example of the consequences of undecidability. Was taught to me in my algorithms and proofs class