A US Department of Justice report was published recently on the interet, with some of its sensitive information hidden from public view. According to Matt Blaze's blog, the Implementation Of The Communications Assistance For Law Enforcement Act By The Federal Bureau Of Investigation report exhibited a classic redaction mistake. Essentially, you can uncover the hidden text by pressing the Ctrl-A, Ctrl-C and then (after opening a text file) Ctrl-V keys. In other words, it's the old select/copy/paste gambit.
Redacting information by obliterating sensitive text with big black lines works sometimes. However, it's easy to get wrong and there have been many cases where those who should know better have attempted to hide data but have failed to do so effectively. Sometimes the results are embarrassing. Other times they have been deadly.