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 <stdio.h>
void main()
{
printf("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.)