Check out the last paragraph of #269 (comment):

"""
Looking over the locks code, I also noticed that async.Semaphore (which can be thought of as a queue without data) uses the approach I sketched above (waking up only one waiter at a time), but without reawakening another waiter when a waiter is cancelled after its future was marked done. So I expect it can be shown to have the same bug.
"""

All we need to do now is write a test that reproduces this issue.