O'Neill In Cambodia
The Cambodia question came down to a question of Kerry's word versus O'Neill's word. All of us here (other than the trolls) were already willing to take Kerry's word over O'Neill's, especially considering the number of lies his group was already caught in.
Finding the tape of O'Neill's statement that he was in Cambodia clinched this issue for Kerry on two counts. It further tipped the balance, if this was necessary, as to who to believe if it should come down to Kerry's word versus O'Neill's word. It also makes Kerry's report of having been in Cambodia sound much more plausible now that we know that O'Neill was there.