Adding this for posterity since I couldn't find anything specific in my Googling endevors.
Problem:
The call is ambiguous between the following methods or properties: 'System.Web.HttpUtility.UrlEncode(string)' and 'System.Web.HttpUtility.UrlEncode(byte[])'
Note: this issue is specific to, at the very least, overloaded methods that accept a string in one signature and a byte[] in a similar signature (see UrlEncode signatures above).
This issue was presenting itself as a RuntimeBinderException in a partial using ASP.NET MVC4 but it could happen in other environments. The key is that UrlEncode has the two overloads and the runtime binder can't figure out which overload to use.