I have an Arduino Uno rev 3. Today i tried to mount a breadboard as a shield for my Arduino on top of it. I plugged in the pin headers i bought and was surprised that the breadboard won't fit. As you can see in the picture below the distance between Vin and A0 is exactly the size of a pin, but the distance between pin 7 and 8 is less than that and so the breadboard won't fit on this side. What's the reasoning behind this (in my eyes) bad design ?
It is a mistake that became a feature for polarizing the shields: forum.arduino.cc/index.php?topic=117441.0
In an 2008 discussion Massimo says:
"I made that mistake, when I made the first arduino board."