In particular a custom property named ["parent"] was not included in
this keying set. This was due to a name collision with the PoseBone.parent
This might not be the correct fix, because I do not know how to make a
custom property non-animatable. Is this possible today? In rB35fedac5
the check for is_animatable was added. That was in 2011, I'm not sure
if the way custom properties are handled changed since then.
Another related commit: rB4d0643a185.