cess blocked sites. We are working on one of those. It does all kinds of fancy things, such as scrambling the text on the source page and using Java script code to unscramble the text and write it. The censoring proxy server cannot block the page unless it parses the Java script to figure out what the actual text is.
To summarize, two points are important. First, a significant percentage of blocked sites have not been reviewed by humans. This situation may be due to honest errors, such as IP address sharing or employees whose eyes are glazing over. But one way or another, significant amounts of content are blocked that should not be. Second, it is easy to circumvent blocking software.