I've noticed problems like this:
Traceback (most recent call last):
File "PyObjCTest/test_array_property.py", line 151, in testGetSetSlice
o.array = l
SystemError: error return without exception set
This appears to be triggered by an exception in a property setter, but only using the pyobjc property class, not regular properties.
This points to a bug in C code (either PyObjC itself or Python, with the former being more likely).