This C function should always return false, but it doesn’t

I stumbled over an interesting question in a forum a long time ago and I want to know the answer.

Consider the following C function:

f1.c

#include <stdbool.h>

bool f1()
{
    int var1 = 1000;
    int var2 = 2000;
    int var3 = var1 + var2;
    return (var3 == 0) ? true : false;
}

This should always return false since var3 == 3000. The main function looks like this:

main.c

#include <stdio.h>
#include <stdbool.h>

int main()
{
    printf( f1() == true ? "true\n" : "false\n");
    if( f1() )
    {
        printf("executed\n");
    }
    return 0;
}

Since f1() should always return false, one would expect the program to print only one false to the screen. But after compiling and running it, executed is also displayed:

$ gcc main.c f1.c -o test
$ ./test
false
executed

Why is that? Does this code have some sort of undefined behavior?

Note: I compiled it with gcc (Ubuntu 4.9.2-10ubuntu13) 4.9.2.