In ASP.NET controls you can use the tilde '~' character to signify the virtual application root of the website, for example when sourcing an image for an ImageButton.
<asp:ImageButton ID="ImageButton1" runat="server" AlternateText="Example Button" ImageUrl="~/images/button.png" />
This is fine for most cases. However, when using URL rewriting (via HttpHandlers) it can become a problem, since ASP.NET will calculate the path to the resource based on the location of the physical
page it is rendering (not the page requesed in the browser [RawUrl]). If the resource is in a directory below the page directory, then it will render a relative path to the HTML, (and not an absolute path from the app root).
When your browser sees a relative path in the HTML it appends that to the current working directory to retreive the resource, which for a page being served up via a rewrite, is probably the wrong directory.
For example, imagine you have the following directory structure:
If you view '/MyButtonPage.aspx' (containing the code from above) the output path to the image would be 'images/button.png'. Your browser will therefore retreive '/images/button.png' which is correct.
However, if you view '/subdir/page-served-by-httphandler-mapping-to-mybuttonpage.aspx' (which rewrites internally the '/MyButtonPage.aspx') then ASP.NET will again render the image path as 'images/button.png' (since it is relative to the 'MyButtonPage.aspx' which was rendered internally). Your browser will now try to retrieve, '/subdir/images/button.png' which does not exist.
A workaround for this problem is to not use the tilde mechanism and instead reference the absolute path to the resource in the code.
<asp:ImageButton ID="ImageButton1" runat="server" AlternateText="Example Button" ImageUrl="/images/button.png" />
However, this will reference from the website root, not the application root, so be careful if your site runs as a virtual application within another site.