**Edit**:
Turns out it's (maybe) not a sign error, but the vertical line in the plus might be hidden by the parenthesis in the background. It still appears like an error!

I noticed this today, on the site header (bottom right of the TeX-logo, both on meta and the main site):

[![](https://i.stack.imgur.com/PcQDb.png)](https://i.stack.imgur.com/PcQDb.png)

Which I read to say

[![enter image description here](https://i.stack.imgur.com/bw2t6.png)](https://i.stack.imgur.com/bw2t6.png)

and it is [wrong](http://www.wolframalpha.com/input/?i=e%5Ex+(cos(y)+%2B+i+sin(y))+%3D+e%5E(x-+i+y)). The [correct expression](http://www.wolframalpha.com/input/?i=e%5E(x-+i+y)+%3D+e%5Ex+(cos(y)+-+i+sin(y))) I believe to be $\cos(-y) + i \sin(-y)$ which simplifies to

[![enter image description here](https://i.stack.imgur.com/U3jAD.png)](https://i.stack.imgur.com/U3jAD.png)

My suggestion would be either to change exp(x-i y) to the more conventional

[![enter image description here](https://i.stack.imgur.com/mr0z2.png)](https://i.stack.imgur.com/mr0z2.png)

(or at least insert the minus sign in the right hand side).