Hello: As it stands, SLIME will error when it encounters compiler notes generated by LispWorks 6 :EXPLAIN functionality. For example: (defun dist (x y) (declare (:explain :variables)) (sqrt (+ (* x x) (* y y)))) will error with #<CONDITIONS:COMPILER-NOTE 402026DC23> fell through ETYPECASE expression. Wanted one of (ERROR STYLE-WARNING WARNING). [Condition of type CONDITIONS:CASE-FAILURE] As a patch, add the two lines prefixed by a + to the function LISPWORKS-SEVERITY in the file "swank-lispworks.lisp" indicated at the end of this mail. Thanks, Robert PATCH FOR "swank-lispworks.lisp": (defun lispworks-severity (condition) (cond ((not condition) :warning) (t (etypecase condition + #+lispworks6 + (conditions:compiler-note :note) (error :error) (style-warning :warning) (warning :warning)))))