I don't know other way to provoke SystemError by Python code. Always if SystemError was leaked this considered a bug and was fixed.

When unload package you need to remove its name and names of its submodules from sys.modules. This is a common case. If the submodule is imported at the same time in other thread you can get SystemError (randomly, with very small probability). I think that if the error can't be avoided, SystemError is a wrong exception for this case.