Comment 5 for bug 1818142

That someone was me. In particular, on SBCL 2.0.10 Linux amd64:

* (progn
    (defclass foo () ())
    (defmethod intialize-instance :after ((o foo) &key) (break))
    (make-instance 'foo))
#<FOO {1002F223B3}>

It took me a while to realize why BREAK is not called and the instance is silently returned.