Here's a typical program assigned to thousands of computer science students each year. Although apparently correct, it contains a error lurking in this simple code.
#include <iostream>
void main()
{
std::cout << "Hello World\n";
}
Hint 1: Every C++ compiler in the world will compile and execute this program without a problem.
Hint 2: The program is non-standard, yet all C++ compiler handle it "correctly". (Gives a new meaning to the term "standard" doesn't it.)