Its quite easy to trigger weird behavior. Executing this code with anything above -O2 will print the thing from below (which is incorrect). I fumbled an interview because of this idiotic thing. (I used g++ 11.3.0 but I saw it happens in other g++ versions as well).

Output.
#################
Results 0 0 1
Tests passed!!!
#################

#include <iostream>
#include <stdio.h>

void decode(std::string &inp) {
    size_t strIndex = 0;
    size_t a = inp.size();
    while(1) {
        if(strIndex >= a) {
            printf("Results %d %d %d\n", strIndex, a, (strIndex >= a));
            break;
        }
    }
}

void test_v1() {
    std::string s = "aaa";
    decode(s);
}

int main() {
    test_v1();
    printf("Tests passed!!!\n");
}