Romerstein was involved with the VENONA intercept decrypts, which was an invaluable service to historians of the Cold War, and to the truth in general. For instance, VENONA confirmed that Alger Hiss really was a Soviet operative.
But Romerstein was guilty of jumping to conclusions in another matter during the early 90s, when excitement about the newly opened Soviet archives was at a peak. He accused left-wing journalist I. F. Stone of working for the Sovs. This charge was swiftly proven to be untenable, if not outright debunked. A fair-use excerpt from The New York Times, Sep 26, 1992. pg. 1.20:
Always let the K.G.B. pick up the lunch tab or risk posthumous vilification. That is seemingly the moral in the sad controversy over I. F. Stone, the left-wing Washington journalist who died in 1989.
Writing in the journal Human Events on June 5, Herbert Romerstein quoted a former K.G.B. agent, Oleg Kalugin, as saying that a well-known American journalist had been a Soviet agent. The journalist, according to Mr. Kalugin, had stopped taking K.G.B. money only after the 1968 Soviet invasion of Czechoslovakia.
Mr. Romerstein added that another former K.G.B. "source," unnamed, had told him the journalist Mr. Kalugin had been referring to was Mr. Stone. The Romerstein account was taken up with unseemly glee by other conservative publications, including The New York Post. But Mr. Kalugin now says: "Never did I mention Stone as a man who was paid as a Soviet agent." So reports Andrew Brown, author of the original article in the London newspaper The Independent that inspired the controversy.
In the current issue of The New York Review of Books, Mr. Brown quotes Mr. Kalugin as saying that his comment about the journalist's refusal to take "any money from us" after 1968 referred solely to the journalist's refusal to allow him to pay for lunch.
1992 was a long time ago, but I'll believe this when someone else provides a lot more corroboration.