Continuity mistake: In the very last scene of the film Paul Newman ignores the ringing phone with Charlotte Rampling on the other end. A wide shot shows a paper coffee cup is on a table to his right, in front of the plastic lid. In close up, he reaches right, picks up the cup from out of shot, drinks from it and, judging by his arm positioning, puts the cup back in the same place. The next, and final, shot of the film is wider angle again. The lid is there, but the cup is now missing.
Character mistake: The mole Laura Fisher is paid off with a check dated Feb 1982. During the trail James Mason says the events happened 4 years ago, in 1976. That would be 6 years ago.
Answer: He couldn't do that. Since the judge ordered her testimony stricken, it was basically like it never happened. Therefore, there was no testimony on which to recall a previous witness.
If Towler were recalled, Galvin could have asked a general question such as "Have you ever asked or ordered anyone to change or alter an admittance form?"
Recalling a witness is something the court (i.e. the judge) has to approve, and given that a) the judge is already biased/bought against Galvin, and b) there is, again, no official testimony upon which to predicate recalling a witness, it is extremely unlikely the judge would permit the doctor to be recalled.