2.2.1 [ECMA-262/5] Section 8.5, The Number Type

C0008:

The specification states:

 In some implementations, external code might be able to detect a difference between various Not-a-Number values, but such behaviour is implementation-dependent; to ECMAScript code, all NaN values are indistinguishable from each other.

IE9 Mode, IE10 Mode, IE11 Mode, and EdgeHTML Mode (All Versions)

NaN values are not normalized to the same value.