Check out this sentence from a Boston Globe article on the fear of nuclear terrorism, and note the effect of the second comma, which I'm pretty sure shouldn't be there:
At least twice since the Sept. 11 attacks, US intelligence officials believed, terrorists had smuggled a nuclear device into the United States, once in New York City and later along the Potomac River near Washington, D.C. A senior Bush administration official who asked not to be identified said that before the information was determined to be unfounded, he considered calling his wife and telling her to take the children and head for the Virginia mountains.
Talk about the difference between the lightning and the lightning bug. Without the comma, on two occasions officials (wrongly) believed there was a loose nuke in the US. With it, at some indeterminate past moment, officials believed that there had been, at different previous times, two loose nukes; and in this alternate reading, they might still believe it. (Hitchens mentioned the second false alarm in a column that can be found here, among other places.)