I'm visiting OWASP for the first time. This comment may apply throughout the site, but this is the first instance I've seen. My suggestion:
Clearly and systematically distinguish counter-examples (ie, examples that demonstrate the insecure way of doing things) from good examples. For example, counter-examples could have a different background color, or at a minimum an obvious comment within the sample itself stating that this is the WRONG WAY TO DO IT! I don't think it's as effective to rely on the surrounding text to clearly identify good from bad examples.
It's not that I think the visitors to this site will generally be idiots, but I think it greatly improves readability and decreases the likelihood of misunderstanding... especially if, as the site grows, an article might provide a number of examples of both the wrong and right way to do things, or if the examples get more lengthy.