>  If the GIL is released before the tuple is fully populated and something access to this tuple via the GC (ex: gc.get_objects()), accessing the tuple can crash, especially in the Python land (for example, repr(the_tuple) is likely to crash).

It can happen even without releasing the GIL: A new tuple is created, then some other object is created using the CAPI, the gc runs, the callback triggers (or the tuplevisit method is invoked) and then kaboom