A: When it's used in Word 2007.
I so wish I was kidding. But apparently the Microsoft Office team have completely lost the plot.
Part of my job is preparing documentation for clients. And part of that documentation is invariably source code or its equivalent (eg configuration files). Most programmers understand the need for layout when displaying, editing and debugging source code (I'll grant that there are some out there who think, "It was hard to write so it should be hard to understand", but they're the minority).
Here's some text in Notepad (not the text that caused me to spot the problem, so no comments on my leet source code management skillz please):
Here's the same text in Word, as a copy + paste as unformatted text:
Note that the text lines up perfectly in Notepad but is screwed up in Word (I've added a 1px red line to help illustrate the difference). Funnily enough the difference in layout varies according to font size (sometimes the centre rows are shorter than the top and bottom rows, sometimes longer).
So could someone please explain to me why Word 2007 now kerns a fixed-width font? Who signed off on that idiocy? And does anyone know how to stop it "helping"?