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):
Which I read to say
My suggestion would be either to change exp(x-i y) to the more conventional
(or at least insert the minus sign in the right hand side).