System.InvalidCastException [A]X cannot be cast to [B]X

I am getting the error below when I run an ASP.NET app. X is a typedDataTableRow defined in reference.cs (proxy for a web service in another project which is not part of the solution). There's only a single Y.dll file in the ASPTemp folder. I restarted Visual studio, cleaned the solution, deleted the ASPTemp folder, deleted all the compiled dll's under the solution, restarted IIS. In the debugger there's a single Y.dll instance in the modules window. I can't determine why I am getting this exception. The two path's in the exception are exactly the same! Somehow the runtime thinks there are two instances of Y.dll. (Y is the name of my project/assembly). I am trying to use Fuslogvw.exe but it's not showing minimal info and nothing useful. Will post another question about it. I am using Visual Studio 2010 in Windows 7 64bit.

One note I am mentioning is that I have two very similar typed datasets. One was created from the other plus new datatables. I use the Merge method to copy the data from one to the other. I did a global rename of the dataset class name in the second ..designer.cs file and I am sure the two datasets are unique. first's class name doesn't exist int the second. I mention this in case the two datasets might be the cause of this but I don't see how.

Any ideas how to troubleshoot this?

Exception Type: System.InvalidCastException
Message: [A]X cannot be cast to [B]X. Type A originates from 'Y, Version=123.10.0.32434,
 Culture=neutral, PublicKeyToken=null' in the context 'Default' at location 
'c:\temp\ASPTemp\xx\bf09df88\62e1519a\assembly\dl3\bdf55bf9\f02959af_c88cce01\Y.dll'. 
Type B originates from 'Y,  Version=123.10.0.32434, Culture=neutral, PublicKeyToken=null' 
in the context 'Default' at location 'c:\temp\ASPTemp\xx\bf09df88\62e1519a\assembly\dl3\bdf55bf9\f02959af_c88cce01\Y.dll'

Solution

I deleted the second dataset and all code that references it as a test and the problem went away. I still do not have an explanation for this behavior when the two datasets are contained in two different classes and there are no compile errors.