We upgraded a javascript library and pointed to the *.min.js files in our MVC4 application. They weren't getting downloaded and were not in the source code when running in debug="true", but worked in debug="false".
The lesson is to remove .min from your js files or the ScriptBundle doesn't return them for downloading.
bundles.Add(new ScriptBundle("~/bundles/x").Include(
"~/Scripts/x*"));