Holy crap. I was about to comment that NANs are not equal to each other, as is the case in every other floating-point representation on the planet[0]. I wasn't actually expecting the page to say either way, but I decided to have a quick look in the vague hope of finding a quote to support that, and instead discovered:

  "nan is equal to itself."

That strikes me as being very odd, and will likely catch a lot of programmers out who are expecting the "normal" behaviour there. I predict many bugs, and much wailing and gnashing of teeth, as a result.

[0] Maybe.

Edit: I see Stormbrew made the point before me, lower down the thread.