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In reporting his series on prominent K Street lobbyist Gerald S.J. Cassidy, Washington Post journo Bob Kaiser reportedly promised his subject: “no surprises.” It’s a common pledge, and a great principle to live by. After all, no profile subject should be blindsided by some stray allegation or innuendo when they’re reading the finished product in the paper, or, in this case, on the Web.

But that’s just what happened last Thursday evening, when Cassidy was reading Chapter 5 of Kaiser’s 27-chapter “Citizen K Street.”

Kaiser recounts a conversation that Cassidy and his wife, Loretta, had over dinner with Cassidy’s then-partner, Kenneth Schlossberg, and his wife, Sophia. Writes Kaiser: “During the meal the Cassidys confided that they had chosen not to have children, that their great ambition in life was to acquire wealth.”

Like any good lobbyist, a sleep-deprived Cassidy went on the offensive. He cranked out a nasty blog item describing “the slash and trash of gotcha journalism” practiced by the Post. Cassidy pointed out that he and his wife couldn’t have their own children and made a tough choice not to adopt.

Top Posties caught the Cassidy blowback. Investigative honcho Jeff Leen got a call, and so did a Post lawyer, according to Tom Alexander, director of corporate communications for Cassidy & Associates.

The complaint was that the Post had gotten it wrong and hadn’t even asked Cassidy about the money-before-children bombshell.

The Post caved on violating the “no surprises” rule but not on the accuracy of its account. Kaiser himself appended a note to the chapter: “In one of our interviews, I raised the issue with Cassidy of ‘your and Loretta’s decision not to have children.’ I did not ask him specifically about the Schlossbergs’ memories of that dinner. I regret that omission.”

Fans of the Kaiser series sniff what went on here. Cassidy and Schlossberg were once partners who founded a successful lobbying operation. Their relationship eventually hit the rocks and they split up. In looking back at key facts from decades ago, Schlossberg’s memory holds up much better than Cassidy’s, as Kaiser’s treatment craftily demonstrates. Check out Schlossberg’s post from last Friday for an audit of his memory bank.

Even the must-have-cash-not-kids moment hasn’t soured the Cassidy establishment on Kaiser: “I think we are cautiously optimistic,” says Alexander. “All along, we trusted, have trusted, and continue to trust Bob and his strong reputation as a thorough and serious journalist.”

Let’s hope this whole thing doesn’t get too lovey-dovey, though. The mudslinging on the Citizen K Street site is just too fun, too insightful. Among my favorite remarks: “I’m sorry to say, but in the battle of Schlossbergs memory versus Cassidy’s memory, the score is 3-0 Schlossberg. His version of events has repeatedly been confirmed by both people and supporting documents. I don’t think many of you understand the ego Cassidy has and his inability to recall the past as it was, not how he rewrote it. No man is going to admit publicly that he didn’t want to have kids. He knows the reaction that brings.”