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Over time, the impromptu Iranian reporters have honed their skills. Some put the date of a skirmish in the file descriptions they send. Others film street signs and landmarks. But the user uploads can sometimes be misleading. Last Wednesday, Mr. Mackey put a call out to readers to determine whether a video was actually new. A commenter pointed to a two-day-old YouTube version.

Cases like this show why the publication of tweets and Flickr photos can be awkward. Echoing others, Mr. Weaver of The Guardian’s blog said his manner of reporting had made some of his colleagues uncomfortable; he recalled one colleague who remarked, “Twitter? I won’t touch it. It’s all garbage.”

On a couple of occasions, The Guardian’s blog featured video clips that were later discovered to be days old. Mr. Weaver said readers of live blogs are “a bit more forgiving” of those incidents, in part because bloggers are transparent about what they do and do not know.

Television anchors were frequently put in the same position while covering Iran. Last Wednesday, the Fox News anchor Shepard Smith showed a YouTube video of police officials beating and dragging people.

“We do not know when or where this video was from,” Mr. Smith told viewers. “We do not even know if it was stag... >>>