Nintendo Entertainment System
The original model Famicom featured two game controllers, both of which were hardwired to the back of the console. The second controller lacked the START and SELECT buttons, but featured a small microphone. Relatively few games made use of this feature. The earliest produced Famicom units initially had square A and B buttons. This was changed to the circular designs because of the square buttons being caught in the controller casing when pressed down and glitches within the hardware causing the system to freeze occasionally while playing a game.

Q: What shape were the A and B buttons on the earliest Famicom controllers?
square