Thanks to the freedom of the press clause in the first amendment, journalists are fairly safe from prosecution in the execution of their duties. Pretty much the only legal issues journalists have to worry about are libel and slander, which are quite difficult cases to win. The libeled/slandered party has to prove first that the statements were false, that the journalist/editor knew the statements were false, and that they intentionally printed/broadcast them anyway, and that by printing/broadcasting them, the journalist caused some harm to come to the plaintiff. There are further hoops to jump through if that person is considered a public figure. So, unless the “off the record” stuff was false, there really aren’t any legal ramifications for a journalist who lets something out that was supposed to be off the record.
However, for the investigative journalist, who relies as much on his reputation as a trustworthy journalist as he does on his writing abilities, losing his credibility can pretty much ensure that no sources will speak to him again—on or off record. Usually, this is a career ending stunt.