What can cause a memory leak in python?

Possible Duplicate:
Python: Is it possible to have an actual memory leak in Python because of your code?

Since the python garbage collector handles detection of circular references (object A referencing Object B and Object B referencing Object A), I was wondering what could cause a memory leak in python code? Can you provide specific examples of code that would create an inaccessible region of memory that the garbage collector could not handle or is such a thing impossible?

Any examples appreciated !