Obscure bug

This lil' MPI program will get its knickers in a twist when run using MPICH. Why? Take a guess.

#include <mpi.h>

void shutdown() {
    MPI_Finalize();
    exit(0);
}

int main (int argc, char ** argv) {
    int w_rank, w_size;

    /* Initialise MPI environment */
    MPI_Init(&argc, &argv);
    MPI_Comm_size(MPI_COMM_WORLD, &w_size);
    MPI_Comm_rank(MPI_COMM_WORLD, &w_rank);

    /* For some reason, we MUST use only 2 procs */
    if (w_size != 2)
    {
        printf("Sorry mate, use only 2 procs\n");
        shutdown();
    }

    /* Do stuff ... */
    printf("Proc %d of %d\n", w_rank, w_size);

    /* Finalise MPI environment */
    shutdown();
}