[bob1029]

I think I still prefer NotImplementedException. It also covers things that are logically reachable but not ready for use yet. The nuance is the string argument to the ctor.

[Deukhoofd]

I do let my IDE register NotImplementedExceptions as TODOs, which is not really desired for things that should not be reachable.

[hardware2win]

Different intention.

[kevingadd]

It's useful to have a separate type since things like exception messages can get optimized out for size in some cases.