I'm in a grumpy mood today, so I decided to take out my frustrations on the ANS for CL, or implementations thereof.  Consider carefully what the following form should return:

(let ((v (make-array 10
    :initial-contents '(0 1 2 3 4 5 6 7 8 9)
    :fill-pointer 5)))
  (loop for x across v
      when (eql x 2) do (incf (fill-pointer v))
      collect x))

ACL and SBCL both return (0 1 2 3 4).  Probably every other implementation does too.  I believe the ANS requires the return to be (0 1 2 3 4 5), although the definition in 6.1.2.1.5 The for-as-across subclause uses various undefined terminology.  (We of X3J13 understood that the loop specification was not our best work.)