If you have ever tried adding a reference to an external assembly in the SalesLogix entity model through the Application Architect and have received the error:

Could not load file or assembly 'file:///Somepath\SomeAssembly.dll' or one of its dependencies. This assembly is built by a runtime newer than the currently loaded runtime and cannot be loaded. (mscorlib)

The problem is that the assembly was compiled in Visual Studio using the wrong .NET framework version.  SalesLogix 7.5.2 only supports .NET 3.5 and prior.  If you build a solution using the >NET 4.0 Framework, the compiled dll will throw this error.