[nomercy400]

Is this like java.lang.UnsupportedOperationException?

[orthoxerox]

No, that's System.NotSupportedException in .NET.

[jug]

Which in turn is not for unreachable or even unexpected cases but expected cases you’re fully aware of, maybe even documented, but that you don’t support.

[orthoxerox]

Yes, like Seek() on a NetworkStream.

[_old_dude_]

In Java, usually an Error like AssertionError is used for those cases.

By example, for a switch on an enum, the compiler inserts a "throw new ...Error()" automatically when the default case is not specified.