Sometimes things you think you know turn out not to be right. I got bitten by this issue recently, and it seemed like something to write down. Because being wrong is fine, as long as you learn something...
There are certain "rules of programming" that I keep hearing about in my career. One that came up in an interesting work debate recently was "you should never use regular expressions to parse HTML". Don't get me wrong - there can be a lot of useful knowledge wrapped up in these rules, but should we always follow them to the letter? I think it's an interesting question...