user19602: 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 ?

user6628: 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."

at http://forum.arduino.cc/index.php?topic=22737.msg171839#msg171839 
with: 

19602: <p>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 ?<a href="https://i.stack.imgur.com/e0R2t.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/e0R2t.jpg" alt="Arduino Uno rev3 with pin header"></a></p>
 6628: <p>It is a mistake that became a feature for polarizing the shields: forum.arduino.cc/index.php?topic=117441.0 </p>

<p>In an <a href="http://forum.arduino.cc/index.php?topic=22737.msg171839#msg171839" rel="nofollow noreferrer">2008 discussion</a> Massimo says: </p>

<blockquote>
  <p>"I made that mistake, when I made the first arduino board."</p>
</blockquote>

<p>at <a href="http://forum.arduino.cc/index.php?topic=22737.msg171839#msg171839" rel="nofollow noreferrer">http://forum.arduino.cc/index.php?topic=22737.msg171839#msg171839</a> 
with: <a href="https://i.stack.imgur.com/wGgQT.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/wGgQT.jpg" alt="enter image description here"></a></p>
