Running this script on CPython works, but crashes on PyPy:

import json
import datetime

from bson.json_util import default, object_hook
from bson.tz_util import utc

data = datetime.datetime(2011, 1, 1, 1, 1, 1, 191000, utc)
data2 = json.loads(json.dumps(data, default=default), object_hook=object_hook)

assert data == data2, (data, data2)

The microseconds (191000) are unserialized to a wrong value (190999).