Why switch_out() may be called twice in a row?

[dmugtasimov]

In a simple setup like this all works fine:

from gevent import monkey; monkey.patch_all()

import time
from collections import deque

import gevent


def func():
    while True:
        print('{:.3f} {} iteration'.format(time.time(), id(gevent.getcurrent())))
        gevent.sleep(0.5)


class CustomGreenlet(gevent.Greenlet):

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

        self.switch_log = deque(maxlen=100)

    def switch(self, *args, **kwargs):
        print('{:.3f} {} switch in'.format(time.time(), id(gevent.getcurrent())))
        self.switch_log.append(0)
        return super().switch(*args, **kwargs)

    def switch_out(self):
        print('{:.3f} {} switch out'.format(time.time(), id(gevent.getcurrent())))
        assert self.switch_log, 'Switch out without switch in is detected'
        assert self.switch_log[-1] == 0, 'Switch out without switch in is detected'
        self.switch_log.append(1)


greenlets = [CustomGreenlet(func) for _ in range(100)]

for greenlet in greenlets:
    greenlet.start()

gevent.joinall(greenlets)

But in a real world applications that involve actual IO and a lot of other stuff I got assert self.switch_log[-1] == 0, 'Switch out without switch in is detected' fired.

2019-01-13 16:06:26,455 ERROR gid:48 ERROR during iteration (in 0.000431776s): TaskResultsConsumer
Traceback (most recent call last):
  File "/home/dmugtasimov/gitrep/packy/packy-agent/packy_agent/worker/loops/base/loop.py", line 38, in iteration_wrapper
    self.iteration()
  File "/home/dmugtasimov/gitrep/packy/packy-agent/packy_agent/worker/loops/base/consumer.py", line 27, in iteration
    item = self.inbound_queue.get(timeout=self.timeout)
  File "src/gevent/queue.py", line 329, in gevent._queue.Queue.get
  File "src/gevent/queue.py", line 344, in gevent._queue.Queue.get
  File "src/gevent/queue.py", line 321, in gevent._queue.Queue.__get_or_peek
  File "src/gevent/_waiter.py", line 151, in gevent.__waiter.Waiter.get
  File "src/gevent/_greenlet_primitives.py", line 60, in gevent.__greenlet_primitives.SwitchOutGreenletWithLoop.switch
  File "src/gevent/_greenlet_primitives.py", line 60, in gevent.__greenlet_primitives.SwitchOutGreenletWithLoop.switch
  File "src/gevent/_greenlet_primitives.py", line 63, in gevent.__greenlet_primitives.SwitchOutGreenletWithLoop.switch
  File "/home/dmugtasimov/gitrep/packy/packy-agent/packy_agent/utils/gevent.py", line 34, in switch_out
    assert self.switch_log[-1][0] == SwitchType.IN, 'Switch out without switch in is detected'
AssertionError: Switch out without switch in is detected

How should I approach to debug this? Is it a bug or feature? Should I ignore consecutive switch_out() calls to the same Greenlet? If so, then which ones?