I admit I didn't know where else to put this, so this seemed appropriate.
In all the pin-out descriptions and tables I have seen, from the first Pi to the most recent (except, of course the Compute Modules), the pins are numbered "incorrectly", which has only just dawned on me while I was trying to layout a PCB using Fritzing.
Conventionally, for an IC or other connector, if it is oriented "correctly", pin#1 is top left, and the numbers increase going down the left side, and increase coming back up on the right.
If for example, you look at the specification for the MCP23008, you can see just this.
However, for the Pi, whether it's a 26 pin or 40 pin header, they go from left to right, then down to the left and right again, and so on.
Now, I recognise that this means that the pin numbering for the 26 pin version and the matching pins on the 40 pin version is the same, which is useful. But it does mean that there is a bit of a mental shift needed when designing a PCB to connect to the Pi, since what is referred to as pin#2 on the Pi (5V) is different to pin#2 on the connector (I2C#1 SDA).
If you had only a list of pin assignments, not a diagram, this could cause much cursing and gnashing of teeth, particularly if you put 5V through the I2C pin.
Of course, the answer is that if you're designing your own PCB, you should double check these things anyway, but out of interest, why was it done this way, against normal convention?
(I know - if it took me 5 years to notice, why am I complaining about it? No, I didn't blow anything up, but it did add an extra layer of "thinking" about the board layout...)