Florian Ebeling:

...

Try it without slime, only in sbcl.

Sure, there it works. (Gives the uppercase A with a small horizontall line above.)

Looks like it relates to some code that transmits this value over network and displays it in emacs.

I remember that I saw something similar in clisp untill I set configured encodings in the both lisp and emacs sides of slime, but I'm not sure exactly.

Lets investigate it.

Try this: (progn (setq ch (code-char 256) nil)

Will it die? I suppose it will not, and you will be able to execute (char-chode ch).

Regards, -Anton