WordPress 2.3 introduced a feature called Canonical URLs. Its purpose is noble - to ensure that your site's content is only available at one URL. The creator of this feature, Mark Jaquith, has written a thorough explanation of the concept, which you should read if you are confused yet still interested.

One issue with the feature is that it can break already-existing redirects in your .htaccess file. For example, when I got tired of hosting the music video for 16 Military Wives and wanted to direct people looking at it to the Internet Archive's page for it, I inserted the following line into .htaccess:
RewriteRule ^video/16militarywives.mp4 http://www.archive.org/details/TheDecemberists16MilitaryWives
Before I upgraded to WordPress 2.3, people who clicked on a link to media.marteydodoo.com/video/16militarywives.mp4 would automatically go to the Internet Archive.

Since I wanted these redirects to keep working, but wanted to keep the cool Canonical URL feature at the same time, I had to figure out a way to modify my custom redirects to keep them working. Looking through the comments on Mark Jaquith's page, it did not seem like anyone had found a solution. Through trial and error, I found that I was able to get it working by using the 'L' parameter, which stops any further redirecting from occurring. The previous redirect, in its new form:
RewriteRule ^video/16militarywives.mp4 http://www.archive.org/details/TheDecemberists16MilitaryWives [L]

Further Reading