Try something like (I hope this comes out OK)

#include

int main(void)
{
int x;
int y[5];

x = 0;
y[5] = 1;

std::cout << x << std::endl;
}

With no optimisation this prints 1, because y[5] does overwrite the memory of x. However, with optimisation turned on, it outputs 0, because the compiler “knows” that x = 0, and just turns the code into:

int main(void)
{ std::cout << 0 << std::endl; }