So you're merrily marching along, happy as a clam because you just set up a glorious new web site that uses, say, a PHP-based content management system like Joomla or WordPress running on a Unix or Linux server. Everything is going well until suddenly, you start to get feedback from users that they can't access certain pages or actions on your site. Worse yet, some people seem to be completely blocked from your site entirely. The problem your users are seeing is a 500 Error, aka "Internal Server Error". Most of the time, for most people, your site works great. But this is becoming a serious issue for some users... What to do?