I just stumbled across the concept of a "Web Garden" in an IIS App Pool. That is, when more than one process serves the same webpage. From what I understand, this means ASP.NET InProc sessions have the same problems as a web farm.
My question is, assuming your production environment is a web farm but your development/test environment is not, would it be helpful to set up a "Web Garden" in dev/test? I'm thinking this would help catch any multi-process/server issues early on, or at least confirm that everything works as expected.