An example of valid C++ code that results in implementation defined behavior at best, undefined at worst.

extra.cpp

#include <iostream>

struct foo {
    double bar;
};

void func(foo x) {
    std::cout << x.bar;
}

main.cpp

#include <iostream>

struct foo {
    long long bar;
};

void func(foo);

int main() {
    foo lol;
    lol.bar = 5;
    func(lol);
}