One reason could be that the part of the code inserting the object uses a different classloader than the code retrieving it.
An instance of a class can not be cast to the same class loaded by a different classloader.
Response to the edit:
What would you do if this happened in production?
This generally happens when the reading and inserting modules each include the same jar containing C1.
Since most containers try the parent classloader first, and then the local classloader (the Parent first strategy), the common solution to the problem is to instead load the class in the closest common parent to the inserting and reading modules.
If you move the module containing the C1 class to the parent module, you force both submodules to get the class from the parent, removing any classloader differences.