Through the Internet, and specifically through the website Snopes.com, people have come to understand a lot better the general concept that stories that are claimed by many reasonable and otherwise trustworthy people to be true are nonetheless false.
I personally learned through Snopes many years ago that a story that I had actually propagated myself was in fact an urban myth. The way it happened is instructive.
One day I was sitting around in an apartment in college when my buddy Tony brought out a tin of cookies. He told a story about how his Mom had sent him these cookies that she had baked from a recipe his aunt had gotten via a long, involved story similar to the one that you can find at this Snopes page.
The cookies, as I recall, were extremely good, as the cookies represented in this story must be if the story is to be plausible. And I went around for quite some time, many years in fact, telling people that I had firsthand knowledge of the story's accuracy. That is, until in 2002 a kind friend pointed me to the aforementioned Snopes page. Ouch.
Of course, you can see pretty clearly in hindsight that what I actually had was fourth-hand knowledge (Tony's Aunt tells Tony's Mom who tells Tony who tells me) of a story I had made zero effort to verify. The reason I felt like I had firsthand knowledge is because I had tasted the cookies. But "I ate some awesome cookies!" is not evidence of anything.
In the case of cookies, these sorts of things are fairly harmless. But in the case of things like alleged Iranian arms smuggling, the consequences actually can be quite dire.
I happen, for whatever reason, to know a lot of people both IRL and via email/IM relationships who work, in some capacity, within the orbit of the Pentagon (in all but one case, it's as contractors, not actual Pentagon personnel.) A LARGE percentage of these people, more than half, have told me some sort of story about how they had firsthand knowledge of the accuracy of administration claims that the Iranian government was smuggling arms into Iraq in support of anti-American fighters there.
It's very frustrating to have these conversations because it's very difficult to find a gentle way of telling someone that despite the fact that I trust and respect them and don't actually think they are lying, nonetheless I give their anecdote zero value and continue to believe that the story their anecdote supports is in fact false.
But at the risk of reopening old wounds, folks, it just isn't the case that the Iranians have any significant role in supplying anti-American fighters in Iraq.
You can see in this article how well-meaning people, having come across some tantalizing-sounding (and, crucially, privileged) piece of data, would be eager to pass along their newfound "knowledge" to others. Obviously there were Iranian arms found in weapons caches used by Iraqi fighters. But without any detail or context, you simply can't draw meaningful conclusions about such information. It's less than useless because it's quasi-information that is nothing but an encouragement for everyone to leap to the same ill-supported conclusion all at once.
We got into Iraq just that way. We were fortunate we didn't get into Iran that way as well. "Data" is not the plural of "anecdote," even when the anecdote is about an official US enemy.