Server path parsed differently based on client configuration

Question

I am working on a Facebook app. It is located at http://apps.facebook.com/rails%5Fdev.

I have a problem. The url path to the images directory gets parsed differently based on which client computer it is called from. If it is called from my development system, it requires the following path:

http://railsacrosseurope.com/public/images

And the following fails:

http://railsacrosseurope.com/public%5Fhtml/public/images

However, if it is called from another client, the path requirements are reversed.

The actual path to the images directory is /public_html/public/images.

I don't understand how my client computer configuration could cause the server to parse this path differently. Does this make sense to anyone?

This is an MVC app which uses URL rewriting (if that helps)