Arduino – fix your pin spacing!September 24th, 2008 by Jeff
… Note that the distance between digital pins 7 and 8 is 160 mil (0.16″), not an even multiple of the 100 mil spacing of the other pins.
Seriously, why??? This means you can’t drop a protoboard on top of an Arduino without a lot of pain dealing with digital pins 8-13.
Of course now it’s too late to switch because everyone that has designed shields for the Arduno made them to fit the weird offset between pins 7 and 8.