Despite being on of our strongest allies (although don't ask Barack Obama about that), the British Navy reportedly recently leaked U.S. Navy military submarine secrets online. The mistake was apparently an accident as a result of a glitch in redacting some government documents. UK's The Guardian explains:
Defence chiefs have been forced into an embarrassing emergency retraction after secret information about the UK's nuclear-powered submarines was inadvertently published on a website.
A technical error meant sections of a Ministry of Defence report that appeared to have been blacked out could in fact be read by anyone who copied and pasted them into another document.
The offending paragraphs have now been properly covered up [PDF] but were reported to have included expert opinion about the fleet's ability to withstand a catastrophic accident.
Details of measures used by the US navy to protect its own nuclear submarines were also accidentally made available, according to the Daily Star Sunday newspaper, which alerted the MoD to the problem. [Emphasis added]
The Conservative MP Patrick Mercer, who served in the army, told the newspaper the information would be "hugely interesting" to Britain's enemies and its release "potentially catastrophic".
The document was an assessment of the British fleet's vulnerability to a nuclear accident.
"As soon as we were told about this, we took steps to ensure the document was removed from the public domain and replaced by a properly redacted version," A British military spokesman told the Guardian. "We take nuclear security very seriously and we are doing everything possible to prevent a recurrence of this."