Did you know about URL Rebasing in MasterPages? - I didn't.
Often, MasterPage and Slave (or ContentPage) are not located in the same directory-level. So you will run into trouble if you have for example images on the MasterPage referenced like this:
<IMG src="images/spice.jpg" />
The ContentPage in a SubDirectory can not resolve the URL and you will get an 404-Error.
What is the Solutions?
Just put an runat="server" and and id="yourID" attribut to the image and the runtime will try to “rebase” relative URLs it finds on server-side controls inside a MasterPage .
This means the following relative path will work, no matter where the MasterPages and ContentPage :
<IMG id="spiceImg" src="images/spice.jpg" runat="server"/>