If you “visibly modify” a key, consequences are unpredictable:

(LET ((hash-table (MAKE-HASH-TABLE :test 'EQUALP)))
  (SETF (GETHASH hash-table hash-table) T)
  (GETHASH hash-table hash-table))
⇒ NIL ;
⇒ NIL

because (SETF GETHASH) modifies hash-table, the very next GETHASH does not find it in itself.