I am trying Prince with a page that has some img tags that are using the double slash notation to inherit whichever protocol (http or https) is being used on the page.

src="//some.site.com/logo.gif"

and Prince spits this out:

prince: //some.site.com/logo.gif: warning: can't open input file: No such file or directory

Is there any reason Prince isn't handling these well?

Is the main document being accessed via a HTTP URL? If it's a local file, then the // won't work.