A mistake by the UK Ministry of Defence resulted in information they had planned to keep secret being made available on the Internet.
Some information, including entire pages, had been blacked out but there was still a way to discover what information had been hidden.
"A technical error meant blacked-out parts of an online MoD report could be read by pasting into another document," reported the BBC.
The Daily Star Sunday, which made the discovery, said it would not publish any information that could jeopardise Britain’s fleet of nuclear subs, but could "reveal the bombshell revelations contained in the secret paragraphs include an expert opinion on exactly how much of a structural failure could cause a deadly core meltdown."
The paper stated that the report, which was written in 2009, described the ability of the fleet’s reactors to cope with another potentially catastrophic accident as “unacceptable.”
There were sections which discussed how American subs deal with on-board disasters.
“This is hugely embarrassing," The Daily Star Sunday quoted a Ministry of Defence source as saying. "Whoever is responsible should be sacked. The Americans will be furious their procedures have been exposed.”
Tory MP Patrick Mercer, a former Army officer, said the information in the document would be very interesting to enemies, and potentially catastrophic.
“It’s a staggeringly stupid thing to do," a senior technology consultant at web safety firm Sophos told The Daily Star Sunday. "Anyone with even an elementary knowledge of computing would know how to read it. I can only assume they gave it to a junior member of staff to deal with.
“If this document is like this, who knows what else is? It’s very sloppy security.”
The report was replaced with a correctly black-out version, but The Telegraph reported that, "after examining a small sample of the thousands of documents published under FOI laws yesterday, this newspaper was able to uncover information that should remain confidential under the law."
That paper said the MoD has now started a review of dozens of documents.