import pickle
import threading

shared = list(range(200))

for trial in range(2000):
    barrier = threading.Barrier(4)

    def dumper():
        try:
            barrier.wait()
            pickle.dumps(shared)
        except Exception:
            pass

    def mutator():
        try:
            barrier.wait()
            for j in range(500):
                shared.append([j] * 20)
                if len(shared) > 100:
                    shared.clear()
                    shared.extend(range(50))
        except Exception:
            pass

    threads = [
        threading.Thread(target=dumper),
        threading.Thread(target=dumper),
        threading.Thread(target=mutator),
        threading.Thread(target=mutator),
    ]
    for t in threads: t.start()
    for t in threads: t.join()

$ python3.14t reproducer.py
Segmentation fault (core dumped)

==2034815==ERROR: AddressSanitizer: SEGV on unknown address 0x00000000000c
    #0 _Py_atomic_load_uint32_relaxed Include/cpython/pyatomic_gcc.h:367
    #1 Py_INCREF Include/refcount.h:267
    #2 batch_list_exact Modules/_pickle.c:3213
    #3 save_list Modules/_pickle.c:3268
    #4 save Modules/_pickle.c:4595
    #5 dump Modules/_pickle.c:4768
    #6 _pickle_dumps_impl Modules/_pickle.c:7992