In the midst of a discussion between the only four people in the world who care about such things, Asbørn Ulsberg writes:
I think the issue of getting plugin developers to author well-formed plugins is solved by getting the core of WordPress to support and enforce XHTML. Getting this right is, as I see it, a two-part enforcement and encouragement battle. First, WordPress should be outputting everything as application/xhtml+xml (including the admin pages) to supporting browsers. [emphasis added]
When he [inserted an invalid Unicode character into my system], I ran into the very problem that Mark mentioned years ago and I had to poke my WordPress database option to switch back to text/html for the WordPress admin panel so I could correct Jacques' invalid character.
Wordpress constitutes a logical system. If errors are introduced into that system, the administration panel is the only place within the system that you can correct those errors. If the administration panel itself does not tolerate the very errors it seeks to correct, then you can not fix the errors without jumping out of the system. This is Logical Systems 101. Go read Gödel, Escher, Bach, then come back and argue in favor of enforcing draconian error handling everywhere.
Sam does not have this problem, because he doesn't have to fix errors from within his system. (In fact, his weblog has no administration panel at all. All of his administration is already done from outside the system.) But your average Wordpress user doesn't have that luxury; they're constrained by either a lack of knowledge (don't know how to jump out of the system) or a lack of privileges (not allowed to jump outside the system). In my original thought experiment, Nick had the latter problem, because he was running on Typepad and had no access to anything outside his own administration pages.
(Interesting postscript: Thought Experiment led to the only documented case in the history of my blog where I changed someone's mind through reasoned argument. Possibly the only documented case in the history of the internet, though I would be interested in other examples if you have them.)