Remember the IRS scandal? Haven't heard much about it lately, have you? Yet for a while, it was big, big news, and so often happens, the initial blockbuster allegations were everywhere, penetrating down to even the least attentive citizen, while the full story, which turned out to be rather less dramatic, got kind of buried. News organizations aren't in the habit of shouting, "BREAKING: That Thing We Said Was Huge Last Week? Eh, Not So Much."
Brendan Nyhan has looked at how this "scandal attention cycle" played out with the IRS and turned it into some charts:
What that means in practice is that while pretty much everybody heard that the IRS was "targeting conservative groups," far fewer people have learned that there is now lots of evidence that the IRS wasn't targeting conservative groups (see Alec MacGillis for a good explanation).
In the news organizations' defense, one could argue that the allegations were pretty dramatic, so they reported on it a lot when they emerged. But then the story didn't have much to sustain it. Had proof emerged that something truly scandalous had happened, then that line would have gone up and stayed up, instead of going up and falling right back down. It isn't that they hid the fact that the Republicans' allegations turned out to have very little support, but you can't write 20 stories saying, "IRS Scandal Still Giant Nothingburger." Once you've said it, to a great degree, you've said it.
That's not completely unreasonable, but it doesn't entirely get them off the hook. There's an analogy with the way they handle corrections. Most media critics agree that a correction, particularly an important one, ought to be given the same prominence as the original mistake. So if your paper runs a front-page story saying Senator Jones is planning to marry a horse, then it turns out that he's actually engaged to a human who enjoys horseback riding, you should probably run that on the front page as well. In practice, of course, that almost never happens; corrections are relegated to an inside page, and nobody reads them.
So it's likely that the idea that "the IRS targeted conservative groups" is going to live pretty much forever, even though it isn't true. And of course, the people who care—conservatives who believe the Obama administration is guilty of pretty much every crime one could imagine—will do their best to keep it alive, facts or no facts.