How to Mess with clos in subtle ways

The code below is wrong and stupid, but can generate very subtle bugs,especially if the class definition and de nstring-capitalize code aren't related

;; STEP 1: Define a class
(defclass foo ()
((xxx :initarg :xxx)))
(make-instance 'foo :xxx 1)

;; STEP 2: use nstring-capitalize in the keyword symbol
(nstring-capitalize (symbol-name :xxx))

;; STEP 3: try to make of instance of class
(make-instance 'foo :xxx 1)

The result is:

Invalid initialization argument: :XXX in call for class #.
[Condition of type SB-PCL::INITARG-ERROR]

See also:
Common Lisp Hyperspec, 7.1.2 [section]

Backtrace:
0: (SB-PCL::CHECK-INITARGS-2-PLIST (:XXX 1) # (:|Xxx|) T)


According to http://www.lispworks.com/documentation/HyperSpec/Body/f_symb_2.htm

"symbol-name returns the name of symbol. The consequences are undefined if name is ever modified."

Shame on me.