1) What happened

the default key for "fire 3" in the input manager is left cmd, which corresponds to the windows key on windows. which is generally the "close this window and open the start menu" button

2) How can we reproduce it using the example you attached

Create a new project,
open the input manager
observe the madness in the "Fire3" drop down.
Would make much more sense to have this as left shift by default

You can use this script to test that it's impossible to use Fire 3 in Unity on Windows:

using UnityEngine;
using System.Collections;

public class NewBehaviourScript : MonoBehaviour
{
void Update()
{
if (Input.GetKeyDown("left cmd"))
print("fire3 key was pressed");

}
}

See also case 575280 that says Alt key doesn't trigger events on Mac