The U.S. Transportation Security Administration (TSA) recently suffered a major intelligence disaster when they posted their Screening Management Standard Operating Procedure manual on their website. This is a classified document, containing details of procedures used to check people entering airports. The classified data was blacked out, but those who did this forgot (or were unaware) that these PDF files can be downloaded and anyone with minimal PDF creation skills can remove the "black out" image to reveal the text beneath.
The data revealed was not all that secret to foreign intelligence agencies, who could have deduced the procedures by simply having operatives observe TSA screeners at work. For example, holders of certain passports (from , Iran, North Korea, Libya, Syria, Sudan, Afghanistan, Lebanon, Somalia, Iraq, Yemen and Algeria) are subject to additional scrutiny. Then there is the list of people who are not screened at all (foreign VIPs and their families, or at least spouses and children under 12). It also confirmed that diplomatic pouches (cargo travelling under diplomatic immunity). Other nations have different exemptions or, often very few. But now, because of an ignorant or ignorant TSA employee, key secrets are secret no more.
This isn't the first time this sort of thing has happened. In several similar incidents, WORD electronic documents were posted, with sensitive information "blacked out" or deleted by a word processing command that can be easily reversed.