Go to:

  http://demos.jquerymobile.com/1.4.2/

and attempt to click on the Transitions link on the bottom of the sidebar.

It takes you to the same page that you were already on, but the url now reads:

http://demos.jquerymobile.com/1.4.2/transitions

If you click transitions again, you get a 404.

It seems that the directory for the 1.4.2 Demo was copied into the Transitions directory and the containing Transitions directory was never given the content.