I've discovered that one of the sites on our web server is having part certain URLs stripped away. Specifically, when a user clicks the link to display images as a slideshow the URL for that page has
?show=slide appended. Just that part is what is being stripped away.
Apart from this problem the site works correctly. Furthermore, the site used to work properly and I'm really not even sure at what point it broke, as I rarely visit it and the only thing that appears in my change log is updates to WordPress and its plugins.
I've confirmed that it's not caused by anything in the .htaccess file by installing an exact replica of the site on my dev server, from where it runs as expected. I've also duplicated the Apache virtualhost config for the site to the test server.
The only significant difference between the live and dev servers is that the former is Centos 5.5 and the other is Debian 6.0.
I'm stuck for ideas. If it's not the Apache virtualhost config or the .htaccess file, where else can I look for what might be causing this?
Although to a mere human it did appear that part of the URL was being eaten up it turned out to be mod-rewrite and a 301 redirect. It was a case of misleading symptoms. The site in question runs WordPress with a number of plugins. The permalink rewriting for the main code and the permalink rewriting for one of the plugins didn't like each other and wouldn't play nicely together.
A bunch of uninstalling and reinstalling, together with turning permalinks off and on in various sequences finally got it all to work properly. As it happens, the part of the URL that was apparently disappearing was wrong anyway, resulting in a broken feature on the site.