They might be thinking of cases like where C compilers can remove code that zeroes memory holding security-sensitive data. [0][1] The compiler is permitted to reason that this memory is about to be deallocated anyway, so we can elide the memset call.

I can't imagine why a C compiler would remove a non-trivial runtime check though (except undefined behaviour).

[0] https://wiki.sei.cmu.edu/confluence/display/c/MSC06-C.+Bewar...

[1] https://lists.isocpp.org/sg14/2020/12/0482.php