If you didn't, hold your ground but listen respectfully. This also is a good time for getting news tips. If someone is upset about a negative story, ask about more positive news happening in his territory. If he says the situation in his office isn't nearly as bad as in another office, ask for details about the other office. Admit your mistakes.
People understand that mistakes happen and they respect people who take responsibility.
Even if you disagree, give the source her say and discuss why you told the story the way you did. Consider whether a follow-up story is warranted. If not, suggest a letter to the editor or op-ed offering. Whatever niceties you engage in to establish rapport, the source should understand that your interest in the relationship is receiving information and understanding. Even if a source spends a lot of time with reporters, he probably doesn't feel completely comfortable facing you and your notebook.