I have created 4 buttons and assigned event to 2 buttons, but when I click on 2 buttons I haven't assigned event yet, an unspecified event has occured

Question

I have an example of a error in my program. I created 4 buttons: 1, 2, 3, 4. Button 2 overlaps button 4, and i just added events for 2 and 4. If I click