[xscott]

Very simple code is UB:

    int handle_untrusted_numbers(int a, int b) {
        if (a < 0) return ERROR_EXPECTED_NON_NEGATIVE;
        if (b < 0) return ERROR_EXPECTED_NON_NEGATIVE;
        int sum = a + b;
        if (sum < 0) {
            return ERROR_INTEGER_OVERFLOW;
        }
        return do_something_important_with(sum);
    }

Every computer you will ever use has two's complement for signed integers, and the standard recently recognized and codified this fact. However, the UB fanatics (heretics) insisted that not allowing signed overflow is an important opportunity for optimizations, so that last if-statement can be deleted by the compiler and your code quietly doesn't check for overflow any more.

There are plenty more examples, but I think this is one of the simplest.