[JoshTriplett]

You can easily do that in a statically typed language as well; you just have to do it explicitly. For instance, Haskell has the error function, of type "String -> a"; you can use it absolutely anywhere, no matter the expected type, give it an error message, and if the value it produces is ever actually used, it throws an exception with the specified message.

[tel]

Also the trace module is pretty useful for the kind of dynamic-debugging practice people often do.

[andrewflnr]

Don't discount the value of doing it implicitly. When you're just throwing stuff together (and there is a place for that), it's a distraction to work around parts of the system that won't work yet, both the typing and the mental overhead.