While figuring out how to deal with unicode i stumbled upon across something which appeared to be killing slime session/sbcl with an ensuing emacs message telling something about process filters.
Urfin from #lisp helped me to narrow this down to evaluation of (format t "~c" #\UD0BB), or otherwise getting #\UD0BB shown on the screen, like having it displayed as the result of evaluation.
It seems to have a 100% repeatability.
I have yesterday's slime and sbcl 1.0/x86/Linux.
regards, Samium Gromoff