A useful test for people who claim to know C: What does this program print? The answer requires a basic understanding of `stdio` and the semantics of forking. The answer is in the comments.

fork.c

#include <stdio.h>
#include <unistd.h>

int main(int argc, char *argv[]) {
    printf("Hello, ");
    fork();
    printf(" World!\n");

    return 0;
}

Build and run:

% gcc -Wall -o fork fork.c && ./fork 
Hello,  World!
Hello,  World!