One failure I've run into is with the use of javascript--for example

<script>

function CNN_getCookies() {
    var hash = new Array;
    if ( document.cookie ) {
        var cookies = document.cookie.split( '; ' );
        for ( var i = 0; i < cookies.length; i++ ) {

......
Note: Notice the "less than" symbol in the javascript above.

</script>

This is some slightly modified source from cnn's site--but the point is
that a "<tag>" pattern can be distinguished, but it's difficult to
differentiate this from a greater than or less than in some enclosed
javascript code.

But even if I were to write some code that could handle this case
effectively I'd probably be dealing with loads of other special cases
within poorly written html source.