Spot the error

 Every now and then I get an email from a reader pointing out an error in one of my books. I'm grateful for  the opportunity to put things right, even though it's hard not to feel like a parent, having a fault in one of your children pointed out. 

The other day, though, I got a correction that felt quite different. The problem was with this illustration from Quantum Computing:

A reader wrote to me to point out (correctly) that the number in the picture should in fact be 0.7071067811... - obvious, I'm sure you're thinking, but I missed it. 

I couldn't help ask if the reader could do amazing mental arithmetic in his head. It turns out not to be the case. He told me that his job was writing graphics software, and this meant that a few key values come up all the time, one of which happens to be this number.

The number will be fixed in future editions. My thanks to Icon's editorial director, Duncan Heath, for pointing me to this Fry and Laurie sketch, which seemed very appropriate...