Why is there an odd gap between pins D7 and D8 on the Uno? The row of pins from D8 and above are offset from the rest, including the opposite row on the analog side. The gap seems to be 0.16" (160 mils) wide, which is not even a multiple or sub-multiple of the standard 0.1".
I've always known about the break in regular spacing but became more conscious of it while designing a PCB to sit on top of the Uno and plug into the female headers.
It's no big deal but I'm curious.
An error;
https://electronics.stackexchange.com/questions/940/arduino-pin-spacing
"Turns out, the actual reason for the odd pin spacing was an error in the original CAD/gerber file. So yea, the odd pin spacing was completely unintentional. Massimo also mentioned that there was a proposal to remove the odd pin spacing starting with the Duemilanove, Mega and future models (a so called "restart"), but was voted down in favor of reverse compatibility with old shields, etc."
Here is the explanation direct from Massimo Banzi (same information as what @srnet already provided, but maybe of interest anyway):
as a benefit this error prevents accidental installation of the shield with an offset of 1 pin 
Aha, so that's it. No evil conspiracy with a sinister purpose behind it. I'm disappointed. 
Or rotated by 180 degrees. It is possible I learned that from personal experience 
You would be surprised. I've seen people manage to insert the old hard disk flat cable connectors the wrong way around. The female connector had a notch, the hard disk male connector had the cut-out for that notch and they still managed to get it wrong.
PC did not boot, oh surprise 
I once saw a documentary about a plane crash, the cause was traced to a fuel valve fitted the wrong way round. The valve had been designed to only fit one way so it couldn't be installed the wrong way, but whoever fitted it found it wouldn't fit (because they had it the wrong way) so they modified it to make it fit.
What really worries me is I know I've occasionally been so convinced I've been doing something right when in reality I've got something exactly wrong that I too have tried to correct something that wasn't wrong to begin with. Luckily I don't work in a safety critical industry.
It may be near impossible to force fit a 9V PP3 connector the wrong way but an attempt could damage the load if there's no protection against reverse polarity.
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