Some websites incorrectly report that Linda Lavin and Van Johnson were replacements in the roles of Daisy Gamble/Melinda and Dr. Mark Bruckner. They did star together in a short tour of the show, but neither Lavin nor Johnson was ever in the show on Broadway.
All ads that we can find from different points during the On a Clear Day run display Barbara Harris and John Cullum as the stars, including the ABCs in the New York Times on June 11, 1966 (the day of the production's closing). Several days before the closing, there was a small article in the Times stating that Johnson would be starring in a tour of the show.
Somewhat surprisingly, an article in the Times on June 13, 1966, stated that other commitments would prevent Van Johnson from starring in the tour, but he did star in the tour, which shut down quickly when the tour's producer declared bankruptcy.
Some sources, including The Best Plays of 1965-1966, incorrectly list Evelyn Page as having played Mrs. Hatch in the opening-night cast. On opening night, Rae Allen played the role. Evelyn Page replaced her shortly into the run.
Edward Jablonski's biography of Alan Jay Lerner states that John Cullum was a member of the show's chorus when he was chosen to replace Louis Jourdan during the tryout run in Boston. Some websites have picked up this incorrect information. Cullum was not in the show prior to taking over the role that Jourdan had been playing, Dr. Mark Bruckner.
The opening-night playbill's list of musical numbers seems not to have completely reflected what was performed on opening night. Our main list of musical numbers for the production comes from a playbill with a revised list from shortly after the opening.
The roles of Bob Brody and Jimmy Dern were cut shortly after opening night. It's even possible that they were gone by opening night, altough they were listed in the opening night playbill.