Every now and then I get an error when running an infopath form talking about trying to load duplicate assemblies. This one always gets me…i clean up my manifest, look at my form xsd, template and sample data to see what i’ve changed. Eventually I find it…the Microsoft.Office.Interop.Infopath.SemiTrust.dll reference must be set to Copy Local = false.
If this is set to true, it will sometimes give you a build error (with no error message of course), rather than the runtime error.