Agree with PP, but then it would have to be noted in the report. In my dd's case there was some subtest in which the tester did not get the sense she had really tried hard enough, she was getting sick, hadn't slept much, etc. Tester decided to give a supplemental test a couple of days later. In her report she indicates her regularly calculated scores and in the written part, she explains the extra subtest, how her scores would be affected, etc and then goes on to explain that it is not clear to the tester which of those scores better reflect her true abilities.
Some testers only go for a substitution if they themselves (tester) messed up pretty badly in giving a certain subtest.
