How can a small change in a code which is never executed make a difference?

Question

I have a very strange problem and since it is not possible for me to send code I will try to explain.

This is more like philosophical problem - I hope someone has time/knowledge to think about this.

1) I have project.cpp looking exactly like this:

#include <pthread.h>
#include <unistd.h>
pthread_t pplayer_thread;
void *play(void*);

int main(int argc, char **argv) {
    pthread_create(&pplayer_thread, NULL, play_cb, NULL);
    usleep(5000000);
    return 0;
}

2) pplayer.cpp looking something like this:

...
void *play_cb(void *arg) {
    // this starts movie using gstreamer and exits thread
}   
...

3) not_executed_from_main.cpp looking something like this:

...
extern MyClass *myObj; // this is included from .h file
...
MyClass *myObj = NULL;
...
some_function() {
    ...
    myObj = MyClass::createNew(args);
    ...
}
...

This is all linked together with various other libraries and ton of garbage, but this is basically what is important.

--> Problem:

When I run this, I should see window playing movie clip using gstreamer for 5 seconds -> BUT I only hear sound!

--> Strange thing:

When I comment the line:

myObj = MyClass::createNew(args);

and run again --> I see gstreamer window also (everything is fine)