Journalism thrives on the factual. As such, it can be fairly said that the single most important rule
is to ensure the accuracy of the facts one is conveying to so many people
In other words, there's no point in coming in first if you're wrong.
You've probably heard of the journalistic fiasco that occured after the 1948 U.S. Presidential election. The Chicago Tribune ran a front page story saying that Thomas Dewey had beaten Harry S. Truman and that his victory was a landslide. The problem was that Truman had won -- the photograph of him holding a copy of the erroneous paper is one of the most famous images in American history. And George Gallup, the publisher who stopped using polls two weeks before the election? His face must have been red.
The pursuit of accuracy, however, is not a responsibility exclusive to those in editorial positions. All writers have to take this into account. If you're interviewing someone, double check the spelling of their name. Triple check. Make sure you have their position or title down correctly. You can never be too careful. The classic example taught by most journalism schools worldwide teaches that if you assume a name is spelt "Smith" it's bound to be "Smythe". Checking your facts saves you and your publication embarrassment later on -- and prevents damage to your credibility as a journalist.
What could happen if you threw caution to the four winds and ignore the first rule of journalism? It depends, really. Depending on the size of your blunder (misspelling the name of some guy you approached for a street interview is reasonably minor, while screwing up a major fact like a date, place name or anything reasonably well known or obvious is more major), your publication might either have to print a retraction (which makes both you and the publication look unprofessional), give you a warning or hand you your walking papers. Had the inaccuracy been major (like had you called John Kerry the official nominee for the Republican Party or said that Al Gore was running for the Democrats in 2004) you run the risk of never being taken seriously in the field again. Or at least not for a while.
The handy thing about the first rule of journalism is that many of its other rules stem from it. It's also directly connected to a key staple of journalism ethics: not printing anything that isn't true. The idea of accuracy is also tied to coherence and clarity; more importantly it discourages writers from commiting the cardinal sin of infusing their work with their opinions.
Such a rule can also easily be applied to the factual part of E2. The fact based nodes are well respected by many and it's entirely possible that people could use it (if they don't use it already) as a resource for factual information. In the same way that the publication of inaccurate information could damage the credibility of a newspaper or magazine, the noding of inaccurate information (even if it's thought to be true) could cause a decrease in the number of people who use E2 as a resource for factual information. It's a good rule to keep in mind while noding facts.