Make an error quine!

Windows Command Prompt

& was unexpected at this time.

Ed (1 byte)

All the other solutions thus far are long and ugly. I suppose that is because of the nature of most error messages. But a good error message is elegant in its simplicity. For that, look no further than ed.

?

Save this to a file called edscript and run with ed < edscript, or run ed<<<?. The result:

?

The question mark is written to stderr and ed returns 1, so this actually is an error message. I wonder why ed isn't very popular?

False (0 bytes)

Run with false filename. It writes the program's source code (i.e. nothing) to stderr and returns 1. Of course, calling false a programming language is questionable, and the zero byte quine is unoriginal, but I thought I might as well add it. There is probably some interpreter for a language that prints no error messages, and could replace false.

Now I wish this was code golf.

CoffeeScript, syntactically valid

As tested on their website using Chrome or Firefox.

ReferenceError: defined is not defined

You can replace defined with anything that's not a built-in variable, but I thought this version was fun. Unfortunately, undefined is not defined in particular doesn't work as a quine.

In CoffeeScript this isn't even a syntax error, because it compiles. This is technically a runtime error in JavaScript, albeit a boring one. CoffeeScript is a likely candidate to produce some more interesting runtime error quines because a lot of funny sentences are valid code. E.g. the above example compiles to

({
  ReferenceError: defined === !defined
});