Rewriting a path with a slash ("/") in it will cause a HyperLink's image specified in the ImageUrl property within an App_Theme file to not be displayed. This can also lead to a "Cannot use a leading ..." error when running the website within IIS.
Here's a screenshot of the issue occuring:
This is kind of a strange bug, since doing the same thing with an Image works just fine. It's only the HyperLink that breaks. I experienced this issue the other day, and it took me a little bit to figure out what was causing the problem. The answer is ASP.NET, kind of.
Download Sample Code Demonstrating this issue here.