I had the problem occur on my system in a very strange way. In my system customers create products that sit inside a directory structure of product categories. So ProductA might sit in the folder CategoryInner inside the folder CategoryOuter. I had just added a feature where my URL would show the category nesting on the URL thusly:
http://www.somedomain.com/product/CategoryOuter/CategoryInner/ProductA.aspx
Obviously the nesting on the URL was just for SEO purposes (and to show the user what category their product was sitting in. But when I used ResolveClientUrl on some URLs that used to work, it must've been confused by the extra fake pathing. The error message was popping up in the debugger on some line that was never the problem so it took me quite some time to figure out what was going on. I wnet through and removed all of my ResolveClientUrl calls that acted on anything that didn't start with a ~ and made the rest of the paths absolute paths.