When compile a file with
(defgeneric test (a b) (:method-combination progn :most-specific-last))
(defmethod test progn (a b) (list a b))
TEST receives a wrong method-combination (mop:generic-function-method-combination #'test) => #<METHOD-COMBINATION STANDARD {6E99175D}>
When defgeneric is called, the generic function TEST is already created by the defmethod macro at macroexpansion time, but not completely initialized, so defgeneric ends up calling reinitialize-instance with :method-combination, but the slot for method-combination is called %METHOD-COMBINATION and its initarg is :%METHOD-COMBINATION, so nothing gets initialized.
This was changed in the following commit: http://trac.common-lisp.net/armedbear/changeset?new=13947%40trunk%2Fabcl%2Fs...