If it happened only once, which it seems was the case with CNN, I wouldn't care. I've seen the mistake made by a number of news organizations. But this isn't the first time it has happened. This isn't the second time it has happened. And this is a network which has taken a deliberate position against this President and the Democratic Party in general. There are a number of occasions on which some Republican has been involved in a big scandal and under his portrait you see John Q. Public (R-State) citing news stories by The Onion as real news in their official blog to make Obama look weak, employing no real liberal voice on any of their opinion shows, and propagating bullshit misinformation like the "terrorist fist bump."
If they mislabeled people like Reagan and Lincoln as Democrats and accidentally listed "Osama Palin" as a guest on one of their shows, then yeah, it wouldn't matter. But virtually all of these slip-ups have been unfavorable to Democrats.