Last week I posed the question, what's wrong with this Outlook icon?
The answer is simple - the slash in the "no" icon goes the wrong way.
My personal keystone for this is a street sign in a certain panel of The Calculus Affair, but the direction of the slash is part of ISO-3864: top-left to bottom-right.
Perhaps a better mnemonic is to think of the word "NO":
The Outlook team isn't the only one to get this wrong, though: