Nothing found issue due to ‘null’ being added to the end of the blog URL

When a post or a page on our site is visited, either directly, via a search engine, or via an internal link, the user is sometimes redirected to a URL with a /null appended.
For example, if the user tries to visit /2009/11/5-must-have-linux-command-line-tools, they are redirected automatically to /2009/11/5-must-have-linux-command-line-tools/null.

This results in our theme telling them “Nothing found for 2009 11 5 must have linux command line tools null“.

This problem is not easily reproducible, but it happens very often (as shown by PMetrics).

Here is a screenshot of the PMetrics page.

I find it almost impossible to reproduce this problem with Google Chrome (dev channel) on Linux, but for my friend using Firefox 3.5.5 on Windows XP regularly experiences the problem, though there seems to be nothing logical behind these occurrences, and, as the PMetrics page tells us, this problem is not limited to Firefox on Windows. It happens for Linux users with Mozilla as well as Safari users on OSX.

Is anyone familiar with this problem?
Where should we start in order to corner it?
If the .htaccess file is of any use, here it is.