Why does `instanceof` error rather than return `false` when used for 2 incompatible classes?

Question

I\'m reading this:
http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.20.2

They say:

Consider the example program

Answer 1

I'd say, because you know at compile-time that it will never be true. Therefore, it's safe to assume this is not what the programmer meant :)

However, there probably is a more java-technical explanation.